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/repository/models.go

@@ -59,18 +59,25 @@ type SongList struct {
 }
 
 type Soundtrack struct {
-	ID             int32      `json:"id"`
-	SoundtrackName string     `json:"soundtrack_name"`
-	Added          time.Time  `json:"added"`
-	Deleted        *time.Time `json:"deleted"`
-	LastChanged    *time.Time `json:"last_changed"`
-	Path           string     `json:"path"`
-	TimesPlayed    int32      `json:"times_played"`
-	LastPlayed     *time.Time `json:"last_played"`
-	NumberOfSongs  int32      `json:"number_of_songs"`
-	Hash           string     `json:"hash"`
-	Uuid           uuid.UUID  `json:"uuid"`
-	Fingerprint    string     `json:"fingerprint"`
+	ID               int32              `json:"id"`
+	SoundtrackName   string             `json:"soundtrack_name"`
+	Added            time.Time          `json:"added"`
+	Deleted          *time.Time         `json:"deleted"`
+	LastChanged      *time.Time         `json:"last_changed"`
+	Path             string             `json:"path"`
+	TimesPlayed      int32              `json:"times_played"`
+	LastPlayed       *time.Time         `json:"last_played"`
+	NumberOfSongs    int32              `json:"number_of_songs"`
+	Hash             string             `json:"hash"`
+	Uuid             uuid.UUID          `json:"uuid"`
+	Fingerprint      string             `json:"fingerprint"`
+	AlternativeNames []string           `json:"alternative_names"`
+	CoverFile        *string            `json:"cover_file"`
+	CoverSource      *string            `json:"cover_source"`
+	CoverExternalID  *string            `json:"cover_external_id"`
+	CoverManual      bool               `json:"cover_manual"`
+	CoverModified    pgtype.Timestamptz `json:"cover_modified"`
+	CoverChecked     pgtype.Timestamptz `json:"cover_checked"`
 }
 
 type Vgmq struct {