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/handlers_test.go

@@ -334,6 +334,8 @@ func (s statsStore) GetAllSoundtracksWithSongs(context.Context) ([]repository.Ge
 	return []repository.GetAllSoundtracksWithSongsRow{{
 		SoundtrackUuid:   statsSoundtrackUUID,
 		SoundtrackName:   "Game A",
+		AlternativeNames: []string{"GA"},
+		CoverModified:    pgtype.Timestamptz{Time: time.Unix(1730000000, 0), Valid: true},
 		SoundtrackPlayed: 4,
 		Songs:            []byte(`[{"song_uuid":"` + statsSongUUID.String() + `","song_name":"Song","times_played":3}]`),
 	}, {
@@ -375,6 +377,11 @@ func TestStatisticsEndpoints(t *testing.T) {
 	assert.Equal(t, int32(4), soundtracks[0].SoundtrackPlayed)
 	assert.Equal(t, "Song", soundtracks[0].Songs[0].SongName)
 	assert.Empty(t, soundtracks[1].Songs, "no songs is an empty list")
+	assert.Equal(t, []string{"GA"}, soundtracks[0].AlternativeNames)
+	require.NotNil(t, soundtracks[0].CoverModified)
+	assert.Equal(t, int64(1730000000), *soundtracks[0].CoverModified)
+	assert.Equal(t, []string{}, soundtracks[1].AlternativeNames, "no names is an empty list")
+	assert.Nil(t, soundtracks[1].CoverModified)
 
 	var summary backend.StatisticsSummary
 	require.NoError(t, json.Unmarshal(e.do(http.MethodGet, "/api/v1/statistics/summary").Body.Bytes(), &summary))