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/src/v1/points/repository.py

@@ -148,7 +148,7 @@ class PointsRepository:
         *,
         email_hash: str,
         user_email_snapshot: str,
-        first_user_id: UUID,
+        first_user_id_snapshot: UUID,
         grant_event_id: str,
     ) -> bool:
         stmt = (
@@ -156,7 +156,7 @@ class PointsRepository:
             .values(
                 email_hash=email_hash,
                 user_email_snapshot=user_email_snapshot,
-                first_user_id=first_user_id,
+                first_user_id_snapshot=first_user_id_snapshot,
                 grant_event_id=grant_event_id,
             )
             .on_conflict_do_nothing(index_elements=[RegisterBonusClaims.email_hash])
@@ -164,3 +164,28 @@ class PointsRepository:
         )
         inserted_id = (await self._session.execute(stmt)).scalar_one_or_none()
         return inserted_id is not None
+
+    async def get_register_bonus_claim(
+        self,
+        *,
+        email_hash: str,
+    ) -> RegisterBonusClaims | None:
+        stmt = (
+            select(RegisterBonusClaims)
+            .where(RegisterBonusClaims.email_hash == email_hash)
+            .limit(1)
+        )
+        return (await self._session.execute(stmt)).scalar_one_or_none()
+
+    async def update_register_bonus_balance_snapshot(
+        self,
+        *,
+        email_hash: str,
+        balance_snapshot: int,
+    ) -> bool:
+        claim = await self.get_register_bonus_claim(email_hash=email_hash)
+        if claim is None:
+            return False
+        claim.balance_snapshot = int(balance_snapshot)
+        await self._session.flush()
+        return True