Add alternative names and cover art for soundtracks

Alternative names (#32):
- PUT /api/v1/soundtracks/{uuid}/alternative-names replaces a soundtrack's
  alternative names (trimmed, deduplicated, without the soundtrack's own name).
- alternative_names is in the soundtrack and statistics responses.
- /find matches them too and shows which name matched.

Covers (#12), stored in COVERS_PATH and off when it is not set:
- GET, PUT (upload) and DELETE /api/v1/soundtracks/{uuid}/cover, and
  PUT .../cover/source to pick a SteamGridDB game or AniList anime by ID.
  Uploaded and picked covers are never replaced automatically.
- With STEAMGRIDDB_API_KEY, a background job searches SteamGridDB, then
  AniList, by name, alternative names and the name without bracketed parts,
  at startup, after a sync and after names change. Search results are ranked
  by matching words, so "Civilization IV" gets "Sid Meier's Civilization IV".
- cover_modified (Unix seconds) is in the soundtrack and statistics responses,
  so clients can cache covers.

The new columns are added to migration 000006, which is not released yet.

Closes #12, closes #32

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/migrations/000006_add_fingerprint_and_queue.down.sql

@@ -1,3 +1,11 @@
+ALTER TABLE soundtrack
+    DROP COLUMN alternative_names,
+    DROP COLUMN cover_file,
+    DROP COLUMN cover_source,
+    DROP COLUMN cover_external_id,
+    DROP COLUMN cover_manual,
+    DROP COLUMN cover_modified,
+    DROP COLUMN cover_checked;
 DROP TABLE queue_song;
 DROP TABLE queue;
 ALTER TABLE soundtrack DROP COLUMN fingerprint;

internal/db/migrations/000006_add_fingerprint_and_queue.up.sql

@@ -27,3 +27,17 @@ CREATE TABLE queue_song (
     added_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     PRIMARY KEY (queue_uuid, song_no)
 );
+
+-- Other names a soundtrack is known by, for search and the cover search.
+ALTER TABLE soundtrack ADD COLUMN alternative_names TEXT[] NOT NULL DEFAULT '{}';
+
+-- Cover art. cover_file is the image's file name in COVERS_PATH, NULL when there is no
+-- cover. Manual covers (uploaded or picked) are never replaced by the background job.
+-- cover_checked is when the job last searched, so soundtracks without a match are not
+-- searched again for a while.
+ALTER TABLE soundtrack ADD COLUMN cover_file TEXT NULL;
+ALTER TABLE soundtrack ADD COLUMN cover_source VARCHAR(20) NULL;
+ALTER TABLE soundtrack ADD COLUMN cover_external_id TEXT NULL;
+ALTER TABLE soundtrack ADD COLUMN cover_manual BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE soundtrack ADD COLUMN cover_modified TIMESTAMPTZ NULL;
+ALTER TABLE soundtrack ADD COLUMN cover_checked TIMESTAMPTZ NULL;