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/search_cache_test.go

@@ -40,28 +40,31 @@ func TestMatchesSubsequence(t *testing.T) {
 }
 
 func TestSearchCache(t *testing.T) {
-	names := []string{"Final Fantasy VII"}
+	soundtracks := []SearchSoundtrack{{Name: "Final Fantasy VII", AlternativeNames: []string{"FF 7"}}}
 	calls := 0
-	cache := NewSearchCache(func(context.Context) ([]string, error) {
+	cache := NewSearchCache(func(context.Context) ([]SearchSoundtrack, error) {
 		calls++
-		return names, nil
+		return soundtracks, nil
 	})
 	ctx := context.Background()
 
 	data, err := cache.Get(ctx)
 	require.NoError(t, err)
-	assert.Equal(t, []SoundtrackData{{Original: "Final Fantasy VII", Lowercase: "final fantasy vii", Cleaned: "finalfantasyvii"}}, data)
+	assert.Equal(t, []SoundtrackData{{
+		SearchName:   SearchName{Original: "Final Fantasy VII", Lowercase: "final fantasy vii", Cleaned: "finalfantasyvii"},
+		Alternatives: []SearchName{{Original: "FF 7", Lowercase: "ff 7", Cleaned: "ff7"}},
+	}}, data)
 	_, _ = cache.Get(ctx)
 	assert.Equal(t, 1, calls, "data is cached")
 
-	names = []string{"Chrono Trigger"}
+	soundtracks = []SearchSoundtrack{{Name: "Chrono Trigger"}}
 	require.NoError(t, cache.Reload(ctx))
 	data, _ = cache.Get(ctx)
 	assert.Equal(t, "Chrono Trigger", data[0].Original)
 }
 
 func TestSearchCacheLoadError(t *testing.T) {
-	cache := NewSearchCache(func(context.Context) ([]string, error) { return nil, errors.New("db down") })
+	cache := NewSearchCache(func(context.Context) ([]SearchSoundtrack, error) { return nil, errors.New("db down") })
 	_, err := cache.Get(context.Background())
 	assert.Error(t, err)
 }