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/queries/soundtrack.sql

@@ -65,3 +65,44 @@ SELECT uuid FROM soundtrack WHERE id = $1;
 
 -- name: AddSoundtrackPlayedByUUID :exec
 UPDATE soundtrack SET times_played = times_played + 1, last_played = now() WHERE uuid = $1;
+
+-- name: SetAlternativeNames :one
+-- Replaces the alternative names. A soundtrack without a cover is searched again with them.
+UPDATE soundtrack
+SET alternative_names = sqlc.arg(names)::text[],
+    cover_checked = CASE WHEN cover_file IS NULL THEN NULL ELSE cover_checked END
+WHERE uuid = sqlc.arg(uuid) AND deleted IS NULL
+RETURNING *;
+
+-- name: ListSoundtracksNeedingCover :many
+-- Active soundtracks without a cover that the job has not searched since checked_before.
+SELECT uuid, soundtrack_name, alternative_names
+FROM soundtrack
+WHERE deleted IS NULL
+  AND cover_file IS NULL
+  AND NOT cover_manual
+  AND (cover_checked IS NULL OR cover_checked < sqlc.arg(checked_before))
+ORDER BY soundtrack_name;
+
+-- name: SetCover :exec
+UPDATE soundtrack
+SET cover_file = sqlc.arg(file),
+    cover_source = sqlc.arg(source),
+    cover_external_id = sqlc.arg(external_id),
+    cover_manual = sqlc.arg(manual),
+    cover_modified = now(),
+    cover_checked = now()
+WHERE uuid = sqlc.arg(uuid);
+
+-- name: MarkCoverChecked :exec
+UPDATE soundtrack SET cover_checked = now() WHERE uuid = $1;
+
+-- name: ClearCover :exec
+-- Removes the cover, so the job searches for one again.
+UPDATE soundtrack
+SET cover_file = NULL, cover_source = NULL, cover_external_id = NULL, cover_manual = false,
+    cover_modified = NULL, cover_checked = NULL
+WHERE uuid = $1;
+
+-- name: ListCoverFiles :many
+SELECT cover_file::text FROM soundtrack WHERE cover_file IS NOT NULL;