feat(profile): add round avatar with upload and cropping

- backend: user_avatar_objects binding (PG+SQLite), GC preserves avatar blobs,
  avatar service (EXIF-stripping WebP 128/512), public avatar read route
- web: avatar upload/crop on profile, round avatar in header/members/comments/author
- android: avatar upload/crop, round avatar on profile and albums header
- openspec: add-user-avatar proposal/design/specs/tasks

backend/internal/photos/keys.go

@@ -15,3 +15,8 @@ func objectKey(objectID, role, ext string) string {
 	}
 	return fmt.Sprintf("objects/%s/%s%s", objectID, role, ext)
 }
+
+// ObjectKey exposes the opaque storage-key format for callers outside the
+// photos package that need to create their own journaled objects (for example
+// user avatars). The key stays entity-free; only the caller knows the binding.
+func ObjectKey(objectID, role, ext string) string { return objectKey(objectID, role, ext) }