fix(reader): persist chapter metadata cache to database

The DetectChapters function in reader.go was serializing chapter detection
results to JSON but then discarding the bytes with `_ = metadataBytes`
instead of writing them to the database. This meant chapter_metadata in
media_items was never populated, forcing re-detection on every request.

Replace the no-op discard with an actual UpdateMediaItemChapterMetadata()
call using the existing sqlc-generated query.

internal/services/reader.go

@@ -105,9 +105,10 @@ func (s *ReaderService) DetectChapters(ctx context.Context, mediaItemID uuid.UUI
 
 	metadataBytes, err := json.Marshal(result)
 	if err == nil {
-		// Update media item with chapter metadata
-		// This would require a new query in database/queries.sql
-		_ = metadataBytes
+		_, _ = s.db.UpdateMediaItemChapterMetadata(ctx, database.UpdateMediaItemChapterMetadataParams{
+			ID:              pgtype.UUID{Bytes: mediaItemID, Valid: true},
+			ChapterMetadata: metadataBytes,
+		})
 	}
 
 	return chapters, nil