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/repo/contract/sqlite_test.go

@@ -44,7 +44,7 @@ func TestSQLiteContract(t *testing.T) {
 		Settings:      settings.NewSQLiteStore(database),
 		Reset: func(ctx context.Context) error {
 			tables := []string{
-				"variant_jobs", "photo_objects", "objects", "photo_tags", "tags",
+				"variant_jobs", "photo_objects", "user_avatar_objects", "objects", "photo_tags", "tags",
 				"refresh_tokens", "share_links", "notifications", "likes", "comments",
 				"invites", "album_members", "photos", "albums", "users", "app_settings",
 			}