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/playlists.sql.go

@@ -173,6 +173,46 @@ func (q *Queries) GetPlaylist(ctx context.Context, id pgtype.UUID) (GetPlaylistR
 	return i, err
 }
 
+const getPlaylistsByIDs = `-- name: GetPlaylistsByIDs :many
+SELECT id, user_id, name, description, is_public, cover_path, track_count, duration_sec, created_at, updated_at, kind, system_variant, seed_artist_id FROM playlists WHERE id = ANY($1::uuid[])
+`
+
+// Batched lookup used by /api/library/sync to hydrate upsert payloads
+// (#357). Mirror of GetArtistsByIDs.
+func (q *Queries) GetPlaylistsByIDs(ctx context.Context, dollar_1 []pgtype.UUID) ([]Playlist, error) {
+	rows, err := q.db.Query(ctx, getPlaylistsByIDs, dollar_1)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []Playlist
+	for rows.Next() {
+		var i Playlist
+		if err := rows.Scan(
+			&i.ID,
+			&i.UserID,
+			&i.Name,
+			&i.Description,
+			&i.IsPublic,
+			&i.CoverPath,
+			&i.TrackCount,
+			&i.DurationSec,
+			&i.CreatedAt,
+			&i.UpdatedAt,
+			&i.Kind,
+			&i.SystemVariant,
+			&i.SeedArtistID,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listAllPlaylistTracksForCollage = `-- name: ListAllPlaylistTracksForCollage :many
 SELECT pt.position,
        albums.cover_art_path AS album_cover_path