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/statistics.go

@@ -15,8 +15,10 @@ type SoundtrackWithSongs struct {
 	SoundtrackID         int32              `json:"soundtrack_id"`
 	SoundtrackUuid       uuid.UUID          `json:"soundtrack_uuid"`
 	SoundtrackName       string             `json:"soundtrack_name"`
+	AlternativeNames     []string           `json:"alternative_names" doc:"Other names the soundtrack is known by"`
 	SoundtrackPlayed     int32              `json:"soundtrack_played"`
 	SoundtrackLastPlayed *time.Time         `json:"soundtrack_last_played,omitempty"`
+	CoverModified        *int64             `json:"cover_modified,omitempty" doc:"When the cover last changed, in Unix seconds. Missing when there is no cover."`
 	Songs                []SongInfoForStats `json:"songs"`
 }
 
@@ -74,8 +76,10 @@ func toSoundtracks[R soundtrackRow](rows []R, err error) ([]SoundtrackWithSongs,
 			SoundtrackID:         row.SoundtrackID,
 			SoundtrackUuid:       row.SoundtrackUuid,
 			SoundtrackName:       row.SoundtrackName,
+			AlternativeNames:     nonNil(row.AlternativeNames),
 			SoundtrackPlayed:     row.SoundtrackPlayed,
 			SoundtrackLastPlayed: row.SoundtrackLastPlayed,
+			CoverModified:        unixSeconds(row.CoverModified),
 			Songs:                parseSongs(row.Songs),
 		})
 	}
@@ -144,7 +148,9 @@ func (s *StatisticsService) NeverPlayedSoundtracks(ctx context.Context) ([]Sound
 			SoundtrackID:     row.SoundtrackID,
 			SoundtrackUuid:   row.SoundtrackUuid,
 			SoundtrackName:   row.SoundtrackName,
+			AlternativeNames: nonNil(row.AlternativeNames),
 			SoundtrackPlayed: row.SoundtrackPlayed,
+			CoverModified:    unixSeconds(row.CoverModified),
 			Songs:            parseSongs(row.Songs),
 		})
 	}