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/migrations/000009_drop_photo_keys.down.sql

@@ -0,0 +1,8 @@
+ALTER TABLE photos
+    ADD COLUMN IF NOT EXISTS storage_key text NOT NULL DEFAULT '',
+    ADD COLUMN IF NOT EXISTS thumb_key text,
+    ADD COLUMN IF NOT EXISTS preview_key text,
+    ADD COLUMN IF NOT EXISTS size bigint NOT NULL DEFAULT 0,
+    ADD COLUMN IF NOT EXISTS width integer NOT NULL DEFAULT 0,
+    ADD COLUMN IF NOT EXISTS height integer NOT NULL DEFAULT 0,
+    ADD COLUMN IF NOT EXISTS mime_type text NOT NULL DEFAULT '';