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/pipeline.go

@@ -2,6 +2,8 @@ package photos
 
 import (
 	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"image"
 	"strconv"
@@ -138,3 +140,9 @@ func buildVariants(data []byte) (thumb, preview []byte, err error) {
 	}
 	return thumbBuf.Bytes(), previewBuf.Bytes(), nil
 }
+
+// checksum returns the hex-encoded SHA-256 of the given bytes.
+func checksum(data []byte) string {
+	sum := sha256.Sum256(data)
+	return hex.EncodeToString(sum[:])
+}