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

backend/src/v1/points/service.py

@@ -92,15 +92,26 @@ class PointsService:
         ).hexdigest()
         event_id = f"register.bonus:{event_hash}"
 
+        claim = await self._repository.get_register_bonus_claim(email_hash=email_hash)
+        account = await self._repository.get_or_create_user_points_for_update(
+            user_id=user_id
+        )
+        if claim is not None and claim.balance_snapshot is not None:
+            account.balance = max(int(claim.balance_snapshot), 0)
+            account.version = int(account.version) + 1
+            return RegisterBonusResult(
+                granted=False,
+                amount=0,
+                balance_after=int(account.balance),
+                event_id=event_id,
+            )
+
         claimed = await self._repository.claim_register_bonus(
             email_hash=email_hash,
             user_email_snapshot=normalized_email,
-            first_user_id=user_id,
+            first_user_id_snapshot=user_id,
             grant_event_id=event_id,
         )
-        account = await self._repository.get_or_create_user_points_for_update(
-            user_id=user_id
-        )
         if not claimed:
             return RegisterBonusResult(
                 granted=False,