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/queries/tracks.sql

@@ -95,3 +95,8 @@ SELECT COUNT(*) FROM tracks WHERE artist_id = $1;
 -- checks the service does next.
 DELETE FROM tracks WHERE id = $1
 RETURNING id, album_id, artist_id, file_path, mbid;
+
+-- name: GetTracksByIDs :many
+-- Batched lookup used by /api/library/sync to hydrate upsert payloads
+-- (#357). Mirror of GetArtistsByIDs.
+SELECT * FROM tracks WHERE id = ANY($1::uuid[]);