feat(photos): separate object storage from metadata layer

Move physical file facts out of the photos table into an objects ledger:

- objects / photo_objects / variant_jobs tables (migration 000008)
- drop storage_key/thumb_key/preview_key/size/width/height/mime_type from
  photos (migration 000009); they are derived via joins to the original object
- album-free storage keys: objects/{id}/{role}.{ext}
- explicit GC via pending/ready/deleting states; objects deleted only when no
  photo references them
- background worker drives variant generation (durable variant_jobs) and GC
- copy/move are metadata-only; storage objects are shared between photos
- remove unused storage.DeletePrefix

Non-destructive to the external JSON API: size/dimensions/mime and URLs are
still returned, just sourced from the object layer.

OpenSpec change: media-object-layer

backend/internal/app/app.go

@@ -1,6 +1,7 @@
 package app
 
 import (
+	"context"
 	"io/fs"
 	"net/http"
 	"strings"
@@ -37,6 +38,7 @@ type Server struct {
 	membersSvc *members.Service
 
 	photos *photos.Service
+	worker *photos.Worker
 
 	social *social.Service
 
@@ -61,6 +63,7 @@ func New(cfg *config.Config, pool *pgxpool.Pool, st *storage.Client, hub *sse.Hu
 
 	photosStore := photos.NewStore(pool)
 	photosSvc := photos.NewService(photosStore, albumsSvc, st, hub)
+	photosWorker := photos.NewWorker(photosSvc, cfg.Worker.Interval, cfg.Worker.Batch, cfg.Worker.PendingTTL)
 
 	socialStore := social.NewStore(pool)
 	socialSvc := social.NewService(socialStore, albumsSvc, photosStore, hub, notifSvc)
@@ -78,12 +81,20 @@ func New(cfg *config.Config, pool *pgxpool.Pool, st *storage.Client, hub *sse.Hu
 		auth:          authSvc,
 		membersSvc:    membersSvc,
 		photos:        photosSvc,
+		worker:        photosWorker,
 		social:        socialSvc,
 		notifications: notifSvc,
 		share:         shareSvc,
 	}
 }
 
+// StartWorker runs the background reconciler until ctx is cancelled.
+func (s *Server) StartWorker(ctx context.Context) {
+	if s.worker != nil {
+		s.worker.Run(ctx)
+	}
+}
+
 func (s *Server) Router() *gin.Engine {
 	r := gin.New()
 	r.Use(gin.Recovery())