Add MCP server with OAuth/API-key auth (#6570)

Adds an optional MCP server (proprietary module) that exposes Stirling's
PDF operations and AI capabilities to MCP clients. Off by default, zero
footprint when disabled.

### What
- New `/mcp` endpoint: streamable-HTTP + JSON-RPC 2.0; 8 tools
(describe_operation, pages/convert/misc/security category tools, AI,
upload, download).
- Runs real operations over an internal loopback; results returned
inline as base64 (small) or by fileId (large).

### Auth (two modes)
- OAuth2 resource server: RFC 9728 protected-resource metadata, RFC 8707
audience binding, JWKS, `mcp.tools.read/write` scopes; binds each token
to a provisioned Stirling account.
- API-key mode: reuses Stirling per-user `X-API-KEY` (no IdP needed).

### Security
- Per-user file ownership in FileStorage: async/queued writes scoped to
the submitting user; legacy/owner-less files stay readable.
- Admin allow/block list controls which operations are exposed.
- Python engine gated behind a shared secret (`X-Engine-Auth`).
- MCP filter chain is isolated and cannot weaken the main app's
security.
- Hardened: no upstream error-body leakage, log injection sanitized,
fileId path/sidecar enumeration blocked.

### Config / footprint
- Disabled by default (`mcp.enabled=false`); all beans
`@ConditionalOnProperty`.
---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

app/common/src/test/java/stirling/software/common/cluster/inprocess/LocalDiskFileStoreTest.java

@@ -3,11 +3,13 @@ package stirling.software.common.cluster.inprocess;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
+import java.nio.file.Files;
 import java.nio.file.Path;
 
 import org.junit.jupiter.api.Test;
@@ -39,4 +41,46 @@ class LocalDiskFileStoreTest {
         assertThrows(IllegalArgumentException.class, () -> store.resolve("a/b"));
         assertThrows(IllegalArgumentException.class, () -> store.resolve("a\\b"));
     }
+
+    @Test
+    void ownerSidecarCannotBeReadAsFileId(@TempDir Path dir) throws IOException {
+        LocalDiskFileStore store = new LocalDiskFileStore(dir.toString());
+        FileStore.Stored stored =
+                store.store(new ByteArrayInputStream("hi".getBytes()), "f.bin", "alice");
+        String sidecarId = stored.fileId() + ".owner";
+        assertThrows(IllegalArgumentException.class, () -> store.resolve(sidecarId));
+        assertThrows(IllegalArgumentException.class, () -> store.retrieveBytes(sidecarId));
+    }
+
+    @Test
+    void ownerIsPersistedAndReturnedByGetOwner(@TempDir Path dir) throws IOException {
+        LocalDiskFileStore store = new LocalDiskFileStore(dir.toString());
+        FileStore.Stored stored =
+                store.store(new ByteArrayInputStream("hi".getBytes()), "f.bin", "alice");
+        assertEquals("alice", store.getOwner(stored.fileId()));
+    }
+
+    @Test
+    void getOwnerReturnsNullWhenNoOwnerWasRecorded(@TempDir Path dir) throws IOException {
+        LocalDiskFileStore store = new LocalDiskFileStore(dir.toString());
+        FileStore.Stored stored =
+                store.store(new ByteArrayInputStream("hi".getBytes()), "f.bin", null);
+        assertNull(store.getOwner(stored.fileId()));
+    }
+
+    @Test
+    void getOwnerReturnsNullForUnknownFileId(@TempDir Path dir) throws IOException {
+        LocalDiskFileStore store = new LocalDiskFileStore(dir.toString());
+        assertNull(store.getOwner("00000000-0000-0000-0000-000000000000"));
+    }
+
+    @Test
+    void deleteRemovesOwnerSidecar(@TempDir Path dir) throws IOException {
+        LocalDiskFileStore store = new LocalDiskFileStore(dir.toString());
+        FileStore.Stored stored =
+                store.store(new ByteArrayInputStream("hi".getBytes()), "f.bin", "alice");
+        assertTrue(store.delete(stored.fileId()));
+        assertFalse(Files.exists(dir.resolve(stored.fileId() + ".owner")));
+        assertNull(store.getOwner(stored.fileId()));
+    }
 }

app/common/src/test/java/stirling/software/common/service/FileStorageDelegationTest.java

@@ -5,6 +5,7 @@ import static org.mockito.Mockito.mock;
 
 import java.io.IOException;
 import java.nio.file.Path;
+import java.util.Optional;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
@@ -19,7 +20,8 @@ class FileStorageDelegationTest {
         FileStorage fs =
                 new FileStorage(
                         mock(FileOrUploadService.class),
-                        new LocalDiskFileStore(tempDir.toString()));
+                        new LocalDiskFileStore(tempDir.toString()),
+                        Optional.empty());
         byte[] payload = "round-trip".getBytes();
         String id = fs.storeBytes(payload, "x.bin");
         assertArrayEquals(payload, fs.retrieveBytes(id));

app/common/src/test/java/stirling/software/common/service/FileStorageOwnershipTest.java

@@ -0,0 +1,107 @@
+package stirling.software.common.service;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.util.Optional;
+import java.util.concurrent.atomic.AtomicReference;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import stirling.software.common.cluster.inprocess.LocalDiskFileStore;
+import stirling.software.common.util.JobContext;
+
+class FileStorageOwnershipTest {
+
+    private FileStorage newStorageWithoutSecurity(Path tempDir) {
+        return new FileStorage(
+                mock(FileOrUploadService.class),
+                new LocalDiskFileStore(tempDir.toString()),
+                Optional.empty());
+    }
+
+    private FileStorage newStorageWithCurrentUser(Path tempDir, AtomicReference<String> userRef) {
+        JobOwnershipService svc = mock(JobOwnershipService.class);
+        when(svc.getCurrentUserId()).thenAnswer(invocation -> Optional.ofNullable(userRef.get()));
+        return new FileStorage(
+                mock(FileOrUploadService.class),
+                new LocalDiskFileStore(tempDir.toString()),
+                Optional.of(svc));
+    }
+
+    @Test
+    void desktopMode_noOwnershipService_storesAndRetrievesWithoutChecks(@TempDir Path tempDir)
+            throws IOException {
+        FileStorage fs = newStorageWithoutSecurity(tempDir);
+        byte[] payload = "desktop".getBytes();
+        String id = fs.storeBytes(payload, "x.bin");
+        assertArrayEquals(payload, fs.retrieveBytes(id));
+    }
+
+    @Test
+    void sameUserStoresAndRetrieves_allowed(@TempDir Path tempDir) throws IOException {
+        AtomicReference<String> user = new AtomicReference<>("alice");
+        FileStorage fs = newStorageWithCurrentUser(tempDir, user);
+        byte[] payload = "alice's file".getBytes();
+        String id = fs.storeBytes(payload, "x.bin");
+        assertArrayEquals(payload, fs.retrieveBytes(id));
+    }
+
+    @Test
+    void differentUserRetrieves_throwsSecurityException(@TempDir Path tempDir) throws IOException {
+        AtomicReference<String> user = new AtomicReference<>("alice");
+        FileStorage fs = newStorageWithCurrentUser(tempDir, user);
+        String id = fs.storeBytes("alice's file".getBytes(), "x.bin");
+        user.set("bob");
+        assertThrows(SecurityException.class, () -> fs.retrieveBytes(id));
+        assertThrows(SecurityException.class, () -> fs.retrieveInputStream(id));
+        assertThrows(SecurityException.class, () -> fs.getFileSize(id));
+        assertThrows(SecurityException.class, () -> fs.fileExists(id));
+        assertThrows(SecurityException.class, () -> fs.deleteFile(id));
+    }
+
+    @Test
+    void anonymousRetrieveOfOwnedFile_allowed_noCurrentUserMeansNoCompare(@TempDir Path tempDir)
+            throws IOException {
+        AtomicReference<String> user = new AtomicReference<>("alice");
+        FileStorage fs = newStorageWithCurrentUser(tempDir, user);
+        byte[] payload = "alice's file".getBytes();
+        String id = fs.storeBytes(payload, "x.bin");
+        user.set(null);
+        assertArrayEquals(payload, fs.retrieveBytes(id));
+    }
+
+    @Test
+    void authedRetrieveOfAnonymousFile_allowed_noOwnerOnFile(@TempDir Path tempDir)
+            throws IOException {
+        AtomicReference<String> user = new AtomicReference<>(null);
+        FileStorage fs = newStorageWithCurrentUser(tempDir, user);
+        byte[] payload = "no-owner".getBytes();
+        String id = fs.storeBytes(payload, "x.bin");
+        user.set("alice");
+        assertArrayEquals(payload, fs.retrieveBytes(id));
+    }
+
+    @Test
+    void propagatedOwner_scopesAsyncWriteWithNoLiveUser(@TempDir Path tempDir) throws IOException {
+        AtomicReference<String> user = new AtomicReference<>(null);
+        FileStorage fs = newStorageWithCurrentUser(tempDir, user);
+        byte[] payload = "alice's async result".getBytes();
+        String id;
+        try {
+            JobContext.setOwner("alice");
+            id = fs.storeBytes(payload, "x.bin");
+        } finally {
+            JobContext.clear();
+        }
+        user.set("alice");
+        assertArrayEquals(payload, fs.retrieveBytes(id));
+        user.set("bob");
+        assertThrows(SecurityException.class, () -> fs.retrieveBytes(id));
+    }
+}

app/common/src/test/java/stirling/software/common/service/FileStorageTest.java

@@ -9,6 +9,8 @@ import java.io.InputStream;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Optional;
+import java.util.UUID;
 import java.util.stream.Stream;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -37,7 +39,10 @@ class FileStorageTest {
     void setUp() throws IOException {
         MockitoAnnotations.openMocks(this);
         fileStorage =
-                new FileStorage(fileOrUploadService, new LocalDiskFileStore(tempDir.toString()));
+                new FileStorage(
+                        fileOrUploadService,
+                        new LocalDiskFileStore(tempDir.toString()),
+                        Optional.empty());
 
         // Create a mock MultipartFile
         mockFile = mock(MultipartFile.class);
@@ -79,7 +84,7 @@ class FileStorageTest {
     void testRetrieveFile() throws IOException {
         // Arrange
         byte[] fileContent = "Test PDF content".getBytes();
-        String fileId = "test-file-1";
+        String fileId = UUID.randomUUID().toString();
         Path filePath = tempDir.resolve(fileId);
         Files.write(filePath, fileContent);
 
@@ -99,7 +104,7 @@ class FileStorageTest {
     void testRetrieveBytes() throws IOException {
         // Arrange
         byte[] fileContent = "Test PDF content".getBytes();
-        String fileId = "test-file-2";
+        String fileId = UUID.randomUUID().toString();
         Path filePath = tempDir.resolve(fileId);
         Files.write(filePath, fileContent);
 
@@ -113,7 +118,7 @@ class FileStorageTest {
     @Test
     void testRetrieveFile_FileNotFound() {
         // Arrange
-        String nonExistentFileId = "non-existent-file";
+        String nonExistentFileId = UUID.randomUUID().toString();
 
         // Act & Assert
         assertThrows(IOException.class, () -> fileStorage.retrieveFile(nonExistentFileId));
@@ -122,7 +127,7 @@ class FileStorageTest {
     @Test
     void testRetrieveBytes_FileNotFound() {
         // Arrange
-        String nonExistentFileId = "non-existent-file";
+        String nonExistentFileId = UUID.randomUUID().toString();
 
         // Act & Assert
         assertThrows(IOException.class, () -> fileStorage.retrieveBytes(nonExistentFileId));
@@ -132,7 +137,7 @@ class FileStorageTest {
     void testDeleteFile() throws IOException {
         // Arrange
         byte[] fileContent = "Test PDF content".getBytes();
-        String fileId = "test-file-3";
+        String fileId = UUID.randomUUID().toString();
         Path filePath = tempDir.resolve(fileId);
         Files.write(filePath, fileContent);
 
@@ -147,7 +152,7 @@ class FileStorageTest {
     @Test
     void testDeleteFile_FileNotFound() {
         // Arrange
-        String nonExistentFileId = "non-existent-file";
+        String nonExistentFileId = UUID.randomUUID().toString();
 
         // Act
         boolean result = fileStorage.deleteFile(nonExistentFileId);
@@ -160,7 +165,7 @@ class FileStorageTest {
     void testFileExists() throws IOException {
         // Arrange
         byte[] fileContent = "Test PDF content".getBytes();
-        String fileId = "test-file-4";
+        String fileId = UUID.randomUUID().toString();
         Path filePath = tempDir.resolve(fileId);
         Files.write(filePath, fileContent);
 
@@ -174,7 +179,7 @@ class FileStorageTest {
     @Test
     void testFileExists_FileNotFound() {
         // Arrange
-        String nonExistentFileId = "non-existent-file";
+        String nonExistentFileId = UUID.randomUUID().toString();
 
         // Act
         boolean result = fileStorage.fileExists(nonExistentFileId);