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/000008_media_objects.up.sql

@@ -0,0 +1,39 @@
+-- Physical objects stored in S3. Keys are opaque and never encode album, photo
+-- or any other metadata entity. The row is the ledger entry that governs the
+-- lifecycle of the corresponding blob.
+CREATE TABLE IF NOT EXISTS objects (
+    id          text PRIMARY KEY,
+    storage_key text NOT NULL UNIQUE,
+    mime_type   text NOT NULL DEFAULT '',
+    size        bigint NOT NULL DEFAULT 0,
+    width       integer NOT NULL DEFAULT 0,
+    height      integer NOT NULL DEFAULT 0,
+    checksum    text NOT NULL DEFAULT '',
+    state       text NOT NULL DEFAULT 'pending'
+        CHECK (state IN ('pending', 'ready', 'deleting')),
+    created_at  timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS objects_state_created_idx ON objects (state, created_at);
+
+-- Binding of a photo to its objects by role. The role is a property of the
+-- binding, not of the object itself.
+CREATE TABLE IF NOT EXISTS photo_objects (
+    photo_id  text NOT NULL REFERENCES photos(id) ON DELETE CASCADE,
+    object_id text NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      text NOT NULL CHECK (role IN ('original', 'thumb', 'preview')),
+    PRIMARY KEY (photo_id, role)
+);
+
+CREATE INDEX IF NOT EXISTS photo_objects_object_idx ON photo_objects (object_id);
+
+-- Durable queue of pending variant generation work, one job per photo.
+CREATE TABLE IF NOT EXISTS variant_jobs (
+    photo_id        text PRIMARY KEY REFERENCES photos(id) ON DELETE CASCADE,
+    attempts        integer NOT NULL DEFAULT 0,
+    next_attempt_at timestamptz NOT NULL DEFAULT now(),
+    last_error      text NOT NULL DEFAULT '',
+    created_at      timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS variant_jobs_due_idx ON variant_jobs (next_attempt_at);