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/sync_test.go

@@ -403,3 +403,34 @@ func TestResetDB(t *testing.T) {
 func TestFormatDuration(t *testing.T) {
 	assert.Equal(t, "01:02:03", formatDuration(time.Hour+2*time.Minute+3*time.Second))
 }
+
+func TestSyncKeepsAlternativeNamesAndCover(t *testing.T) {
+	f := newSyncFixture(t)
+	f.write("Final Fantasy 7/01.mp3", "a1")
+	f.run(false)
+	st := f.soundtrack("Final Fantasy 7")
+	_, err := f.store.SetAlternativeNames(context.Background(),
+		repository.SetAlternativeNamesParams{Uuid: st.Uuid, Names: []string{"FF7"}})
+	require.NoError(t, err)
+	file, source := "cover.jpg", "upload"
+	require.NoError(t, f.store.SetCover(context.Background(),
+		repository.SetCoverParams{Uuid: st.Uuid, File: &file, Source: &source, Manual: true}))
+
+	// Renamed: the same row is kept.
+	require.NoError(t, os.Rename(filepath.Join(f.dir, "Final Fantasy 7"), filepath.Join(f.dir, "Final Fantasy VII")))
+	f.run(false)
+	renamed := f.soundtrack("Final Fantasy VII")
+	assert.Equal(t, st.Uuid, renamed.Uuid)
+	assert.Equal(t, []string{"FF7"}, renamed.AlternativeNames)
+	assert.Equal(t, &file, renamed.CoverFile)
+
+	// Removed and added again.
+	require.NoError(t, os.Rename(filepath.Join(f.dir, "Final Fantasy VII"), filepath.Join(t.TempDir(), "away")))
+	f.run(false)
+	require.NoError(t, os.MkdirAll(filepath.Join(f.dir, "Final Fantasy VII"), 0o755))
+	f.write("Final Fantasy VII/01.mp3", "a1")
+	f.run(false)
+	readded := f.soundtrack("Final Fantasy VII")
+	assert.Nil(t, readded.Deleted)
+	assert.Equal(t, []string{"FF7"}, readded.AlternativeNames)
+}