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/backend/music.go

@@ -348,6 +348,19 @@ func (m *MusicService) SoundtrackNames(ctx context.Context) ([]string, error) {
 	return names, nil
 }
 
+// SearchSoundtracks returns every soundtrack's name and alternative names, for search.
+func (m *MusicService) SearchSoundtracks(ctx context.Context) ([]SearchSoundtrack, error) {
+	soundtracks, err := m.allSoundtracks(ctx)
+	if err != nil {
+		return nil, err
+	}
+	result := make([]SearchSoundtrack, len(soundtracks))
+	for i, s := range soundtracks {
+		result[i] = SearchSoundtrack{Name: s.SoundtrackName, AlternativeNames: s.AlternativeNames}
+	}
+	return result, nil
+}
+
 // SoundtrackNamesRandom returns all soundtrack names in random order.
 func (m *MusicService) SoundtrackNamesRandom(ctx context.Context) ([]string, error) {
 	names, err := m.SoundtrackNames(ctx)