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/testutil/fakedb/fakedb.go

@@ -193,6 +193,93 @@ func (s *Store) GetSoundtrackByUUID(_ context.Context, id uuid.UUID) (repository
 	return repository.Soundtrack{}, pgx.ErrNoRows
 }
 
+func (s *Store) soundtrackByUUID(id uuid.UUID) int {
+	return slices.IndexFunc(s.Soundtracks, func(st repository.Soundtrack) bool { return st.Uuid == id })
+}
+
+func (s *Store) updateSoundtrackByUUID(id uuid.UUID, f func(*repository.Soundtrack)) error {
+	defer s.lock()()
+	if s.Err != nil {
+		return s.Err
+	}
+	if i := s.soundtrackByUUID(id); i >= 0 {
+		f(&s.Soundtracks[i])
+	}
+	return nil
+}
+
+func (s *Store) SetAlternativeNames(_ context.Context, arg repository.SetAlternativeNamesParams) (repository.Soundtrack, error) {
+	defer s.lock()()
+	if s.Err != nil {
+		return repository.Soundtrack{}, s.Err
+	}
+	i := s.soundtrackByUUID(arg.Uuid)
+	if i < 0 || s.Soundtracks[i].Deleted != nil {
+		return repository.Soundtrack{}, pgx.ErrNoRows
+	}
+	st := &s.Soundtracks[i]
+	st.AlternativeNames = slices.Clone(arg.Names)
+	if st.CoverFile == nil {
+		st.CoverChecked = pgtype.Timestamptz{}
+	}
+	return *st, nil
+}
+
+func (s *Store) ListSoundtracksNeedingCover(
+	_ context.Context, checkedBefore pgtype.Timestamptz,
+) ([]repository.ListSoundtracksNeedingCoverRow, error) {
+	defer s.lock()()
+	if s.Err != nil {
+		return nil, s.Err
+	}
+	var rows []repository.ListSoundtracksNeedingCoverRow
+	for _, st := range sortedByName(slices.Clone(s.Soundtracks)) {
+		if st.Deleted != nil || st.CoverFile != nil || st.CoverManual ||
+			(st.CoverChecked.Valid && !st.CoverChecked.Time.Before(checkedBefore.Time)) {
+			continue
+		}
+		rows = append(rows, repository.ListSoundtracksNeedingCoverRow{
+			Uuid: st.Uuid, SoundtrackName: st.SoundtrackName, AlternativeNames: st.AlternativeNames,
+		})
+	}
+	return rows, nil
+}
+
+func (s *Store) SetCover(_ context.Context, arg repository.SetCoverParams) error {
+	return s.updateSoundtrackByUUID(arg.Uuid, func(st *repository.Soundtrack) {
+		now := pgtype.Timestamptz{Time: time.Now(), Valid: true}
+		st.CoverFile, st.CoverSource, st.CoverExternalID = arg.File, arg.Source, arg.ExternalID
+		st.CoverManual, st.CoverModified, st.CoverChecked = arg.Manual, now, now
+	})
+}
+
+func (s *Store) MarkCoverChecked(_ context.Context, id uuid.UUID) error {
+	return s.updateSoundtrackByUUID(id, func(st *repository.Soundtrack) {
+		st.CoverChecked = pgtype.Timestamptz{Time: time.Now(), Valid: true}
+	})
+}
+
+func (s *Store) ClearCover(_ context.Context, id uuid.UUID) error {
+	return s.updateSoundtrackByUUID(id, func(st *repository.Soundtrack) {
+		st.CoverFile, st.CoverSource, st.CoverExternalID, st.CoverManual = nil, nil, nil, false
+		st.CoverModified, st.CoverChecked = pgtype.Timestamptz{}, pgtype.Timestamptz{}
+	})
+}
+
+func (s *Store) ListCoverFiles(context.Context) ([]string, error) {
+	defer s.lock()()
+	if s.Err != nil {
+		return nil, s.Err
+	}
+	var files []string
+	for _, st := range s.Soundtracks {
+		if st.CoverFile != nil {
+			files = append(files, *st.CoverFile)
+		}
+	}
+	return files, nil
+}
+
 func (s *Store) MarkSoundtracksDeletedExcept(_ context.Context, ids []int32) error {
 	defer s.lock()()
 	if s.Err != nil {