feat: album covers with palettes and signed links (M2)

- covers are taken from the embedded picture of the first track or a
  cover/folder/front image in the album folder, converted by ffmpeg to
  WebP 64/300/1200 and named by content hash, so albums sharing a cover
  share the files; a full scan looks again for albums without one
- a 3-colour palette (dominant, second, most saturated accent) from a
  deterministic k-means feeds placeholders, page tints and the wave
- /covers/{id} is public but HMAC-signed and immutable; nginx serves
  the file via X-Accel-Redirect, X_ACCEL=false serves it from api
- the web client picks the size by display size and pixel density
- install.sh and update.sh generate STREAM_SIGNING_KEY

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/internal/search/service.go

@@ -24,6 +24,7 @@ const (
 
 // Handler реализует search-часть gen.StrictServerInterface.
 type Handler struct {
+	dto     library.DTO
 	meili   *Meili
 	catalog *library.Store
 	indexer *Indexer
@@ -31,8 +32,8 @@ type Handler struct {
 }
 
 // NewHandler создаёт обработчик поиска.
-func NewHandler(meili *Meili, catalog *library.Store, indexer *Indexer, log *slog.Logger) *Handler {
-	return &Handler{meili: meili, catalog: catalog, indexer: indexer, log: log}
+func NewHandler(meili *Meili, catalog *library.Store, indexer *Indexer, dto library.DTO, log *slog.Logger) *Handler {
+	return &Handler{meili: meili, catalog: catalog, indexer: indexer, dto: dto, log: log}
 }
 
 type limits struct{ artists, albums, tracks, offset int }
@@ -132,7 +133,7 @@ func (h *Handler) hydrate(ctx context.Context, f found, corrected string) (gen.S
 			return out, err
 		}
 		for _, a := range rows {
-			out.Albums = append(out.Albums, library.AlbumSummaryDTO(a))
+			out.Albums = append(out.Albums, h.dto.AlbumSummary(a))
 		}
 	}
 	if len(f.tracks) > 0 {
@@ -141,7 +142,7 @@ func (h *Handler) hydrate(ctx context.Context, f found, corrected string) (gen.S
 			return out, err
 		}
 		for _, t := range rows {
-			out.Tracks = append(out.Tracks, library.TrackDTO(t))
+			out.Tracks = append(out.Tracks, h.dto.Track(t))
 		}
 	}
 	return out, nil