fix: preserve points balance across account re-registration

Persist a per-email balance snapshot before account deletion and restore it on same-email re-registration, preventing both unintended balance reset and repeated signup bonus grants.

backend/tests/unit/test_user_service_delete_account.py

@@ -10,7 +10,7 @@ from v1.users.service import UserService
 
 
 class _NoopRepository:
-    pass
+    session = None
 
 
 class _FakeStorage:
@@ -28,7 +28,7 @@ class _FakeStorage:
 
 @pytest.mark.asyncio
 async def test_delete_account_success_calls_storage_cleanup_and_auth_delete() -> None:
-    user = CurrentUser(id=uuid4(), email="test@example.com")
+    user = CurrentUser(id=uuid4(), email=None)
     storage = _FakeStorage()
     service = UserService(
         current_user=user,
@@ -46,7 +46,7 @@ async def test_delete_account_success_calls_storage_cleanup_and_auth_delete() ->
 async def test_delete_account_raises_profile_delete_failed_on_storage_cleanup_error() -> (
     None
 ):
-    user = CurrentUser(id=uuid4(), email="test@example.com")
+    user = CurrentUser(id=uuid4(), email=None)
 
     class _FailingStorage(_FakeStorage):
         async def delete_prefix(self, *, bucket: str, prefix: str) -> int:
@@ -72,7 +72,7 @@ async def test_delete_account_raises_profile_delete_failed_on_storage_cleanup_er
 async def test_delete_account_raises_profile_delete_failed_on_auth_delete_error() -> (
     None
 ):
-    user = CurrentUser(id=uuid4(), email="test@example.com")
+    user = CurrentUser(id=uuid4(), email=None)
 
     class _FailingStorage(_FakeStorage):
         async def delete_auth_user(self, *, user_id: str) -> None: