perf(library): resolve album and genre file paths in one query

"Play this artist" awaited `GetAlbumTracks` inside a for loop — 13
sequential round trips for a 12-album artist — and every one of the
four sites doing that asked for whole track rows to read `FilePath`
off them. Five genres cost 6 MB across the IPC.

`GetFilePathsByAlbums(ids, libraryID)` and `GetFilePathsByGenres(names,
libraryID)` answer once and carry only the paths. Measured at 50 000
tracks: an artist 13 calls / 74.2 kB -> 2 / 19.2 kB, twenty albums
20 / 117.5 kB / 7.8 ms -> 1 / 26.0 kB / 1.7 ms, five genres
5 / 6 014 kB / 213 ms -> 1 / 1 291 kB / 32.6 ms, with the returned path
lists identical.

They return the paths grouped by album id or genre name rather than
flattened, because the caller owns the order — an album list is sorted
by name, not by id, and a flattened result would silently reorder a
queue — and because the album drag cache stores them per album. A
libraryID of 0 means "every library", matching an unset filter.

backend/database/sql/queries/audio_files.sql

@@ -295,3 +295,26 @@ LEFT JOIN release_groups rg ON rgr.release_group_id = rg.id
 LEFT JOIN file_types ft ON af.file_type_id = ft.id
 WHERE rgr.release_group_id = ? AND af.library_id = ?
 ORDER BY rgr.disc_number, rgr.track_number;
+
+-- "Play this artist" and "play these albums" wanted file paths and asked
+-- for whole track rows to get them, one round trip per album (perf.m2).
+-- These answer the same question in one query and carry only what the
+-- caller uses; the release group id comes back so the caller can keep
+-- its own album ordering.
+
+-- name: GetFilePathsByReleaseGroups :many
+SELECT rgr.release_group_id, af.file_path
+FROM release_group_recordings rgr
+JOIN recordings r ON rgr.recording_id = r.id
+JOIN audio_files af ON af.recording_id = r.id
+WHERE rgr.release_group_id IN (sqlc.slice('release_group_ids'))
+ORDER BY rgr.disc_number, rgr.track_number;
+
+-- name: GetFilePathsByReleaseGroupsByLibrary :many
+SELECT rgr.release_group_id, af.file_path
+FROM release_group_recordings rgr
+JOIN recordings r ON rgr.recording_id = r.id
+JOIN audio_files af ON af.recording_id = r.id
+WHERE rgr.release_group_id IN (sqlc.slice('release_group_ids'))
+  AND af.library_id = ?
+ORDER BY rgr.disc_number, rgr.track_number;