feat(server): GET /api/library/sync endpoint

Returns batched upserts + deletes since the supplied cursor. Empty cursor
returns full snapshot; subsequent calls pull deltas. Per-user entities
(likes, playlists) are scoped to the authed user. Composite-key entities
(likes, playlist_tracks) use stable string ids encoded by sync.EncodeLikeID
/ sync.EncodePlaylistTrackID.

Behavior:
  204 No Content - no changes since cursor
  200 OK         - JSON syncResponse {cursor, upserts, deletes}
  410 Gone       - cursor older than oldest log row; client must reset
  401 / 500      - standard envelope errors

Adds sqlc queries GetAlbumsByIDs, GetTracksByIDs, GetPlaylistsByIDs to
mirror the existing GetArtistsByIDs.

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

internal/db/dbq/tracks.sql.go

@@ -143,6 +143,48 @@ func (q *Queries) GetTrackByPath(ctx context.Context, filePath string) (Track, e
 	return i, err
 }
 
+const getTracksByIDs = `-- name: GetTracksByIDs :many
+SELECT id, title, album_id, artist_id, track_number, disc_number, duration_ms, file_path, file_size, file_format, bitrate, mbid, genre, added_at, updated_at FROM tracks WHERE id = ANY($1::uuid[])
+`
+
+// Batched lookup used by /api/library/sync to hydrate upsert payloads
+// (#357). Mirror of GetArtistsByIDs.
+func (q *Queries) GetTracksByIDs(ctx context.Context, dollar_1 []pgtype.UUID) ([]Track, error) {
+	rows, err := q.db.Query(ctx, getTracksByIDs, dollar_1)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []Track
+	for rows.Next() {
+		var i Track
+		if err := rows.Scan(
+			&i.ID,
+			&i.Title,
+			&i.AlbumID,
+			&i.ArtistID,
+			&i.TrackNumber,
+			&i.DiscNumber,
+			&i.DurationMs,
+			&i.FilePath,
+			&i.FileSize,
+			&i.FileFormat,
+			&i.Bitrate,
+			&i.Mbid,
+			&i.Genre,
+			&i.AddedAt,
+			&i.UpdatedAt,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listArtistTracksForUser = `-- name: ListArtistTracksForUser :many
 SELECT t.id, t.title, t.album_id, t.artist_id, t.track_number, t.disc_number, t.duration_ms, t.file_path, t.file_size, t.file_format, t.bitrate, t.mbid, t.genre, t.added_at, t.updated_at,
        albums.title AS album_title,