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/photos/keys.go

@@ -0,0 +1,17 @@
+package photos
+
+import (
+	"fmt"
+	"strings"
+)
+
+// objectKey builds an opaque, album-free storage key for a physical object.
+// The key never encodes album, photo or any other metadata entity; the mapping
+// between a key and its photo binding lives only in the metadata layer. The
+// role segment is cosmetic (for debugging) and is not authoritative.
+func objectKey(objectID, role, ext string) string {
+	if ext != "" && !strings.HasPrefix(ext, ".") {
+		ext = "." + ext
+	}
+	return fmt.Sprintf("objects/%s/%s%s", objectID, role, ext)
+}