test(naming): document bare-id trade-off; cover CJK title + movies numeric

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

internal/naming/folderid.go

@@ -69,6 +69,9 @@ func ParseFolderIDs(folderName string, folderType string) *FolderIDHints {
 
 	// A bare trailing number is only an ID when appended to a real title. If the
 	// name has no letters (e.g. "86", "22 7"), it's a numeric title, not an ID.
+	// Trade-off: a folder named ONLY a bare provider id (e.g. "81189") with no
+	// title text now returns nil rather than that id — acceptable because
+	// Sonarr/Radarr never produce bare-number folders.
 	if !containsLetter(trimmed) {
 		return nil
 	}

internal/naming/folderid_numeric_test.go

@@ -22,4 +22,16 @@ func TestParseFolderIDs_NumericTitleIsNotAnID(t *testing.T) {
 	if got == nil || got.TmdbID != "27205" {
 		t.Errorf(`ParseFolderIDs("{tmdb-27205}","movies") = %+v, want TmdbID="27205"`, got)
 	}
+
+	// Non-Latin (CJK) title with a trailing real id must still parse —
+	// unicode.IsLetter covers kana/kanji, so this is treated as title + id.
+	got = ParseFolderIDs("進撃の巨人 73743", "series")
+	if got == nil || got.TvdbID != "73743" {
+		t.Errorf(`ParseFolderIDs("進撃の巨人 73743","series") = %+v, want TvdbID="73743"`, got)
+	}
+
+	// Numeric-only title for a movies library is also not an id.
+	if got := ParseFolderIDs("86", "movies"); got != nil {
+		t.Errorf(`ParseFolderIDs("86","movies") = %+v, want nil`, got)
+	}
 }