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/server/api.go

@@ -146,8 +146,10 @@ func apiError(err error) error {
 	switch {
 	case errors.Is(err, backend.ErrQueueEmpty), errors.Is(err, backend.ErrNoSongs), errors.Is(err, backend.ErrNotFound):
 		return huma.Error404NotFound(err.Error())
-	case errors.Is(err, backend.ErrInvalidName):
+	case errors.Is(err, backend.ErrInvalidName), errors.Is(err, backend.ErrInvalidImage):
 		return huma.Error400BadRequest(err.Error())
+	case errors.Is(err, backend.ErrCoversDisabled):
+		return huma.Error503ServiceUnavailable(err.Error())
 	case errors.Is(err, backend.ErrUpstream):
 		return huma.Error502BadGateway(err.Error())
 	default: