fix(audiobooks): emit media.tags so ABS Android downloads parse

The ABS Android client's Kotlin Book model declares
    var tags: List<String>  // non-nullable
and Jackson throws MissingKotlinParameterException when the field is
absent from /api/items/{id} responses. ApiHandler.getLibraryItem
WithProgress catches the exception and resolves cb(null), at which
point AbsDownloader silently aborts — the user sees no error, the
download just never starts.

Add Tags to LibraryItemMedia and initialise it to []string{} at
construction. Verified end-to-end: download URL the Android client
builds (/api/items/{id}/file/{ino}/download) returns HTTP 206 with
audio/mpeg bytes; the parent /api/items/{id} response now carries
"tags": [] so Jackson deserialises cleanly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/audiobooks/abs/libraries_handler.go

@@ -571,6 +571,7 @@ func siloItemToLibraryItem(item *models.MediaItem, lib AudiobookLibrary, baseURL
 			Tracks:     []AudioTrack{},
 			Chapters:   []ChapterABS{},
 			NumTracks:  0, // populated by item-detail handler
+			Tags:       []string{},
 		},
 		AddedAt:   addedAtMs,
 		UpdatedAt: updatedAtMs,

internal/audiobooks/abs/types.go

@@ -121,6 +121,13 @@ type LibraryItemMedia struct {
 	Tracks     []AudioTrack `json:"tracks"`
 	Chapters   []ChapterABS `json:"chapters"`
 	NumTracks  int          `json:"numTracks"`
+	// Tags is a book-level tag list. NEVER null on the wire — the ABS
+	// Android client's Kotlin `Book.tags: List<String>` is non-nullable,
+	// so Jackson throws MissingKotlinParameterException when the field
+	// is absent (or null) and the entire LibraryItem fails to parse —
+	// which silently breaks downloads (the downloader's apiHandler
+	// callback receives null and gives up). Always emit []  in v1.
+	Tags []string `json:"tags"`
 }
 
 // CollapsedSeriesV1 is the per-item annotation real ABS attaches when