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/integration/test_register_run_delete_reregister.py

@@ -170,7 +170,7 @@ async def test_register_run_delete_reregister_keeps_bonus_single_use(
     )
     reregister_balance.raise_for_status()
     re_data = reregister_balance.json()
-    assert int(re_data["balance"]) == 0
+    assert int(re_data["balance"]) == int(after_data["balance"])
 
     async with AsyncSessionLocal() as session:
         points2 = (
@@ -217,3 +217,54 @@ async def test_register_run_delete_reregister_keeps_bonus_single_use(
             ).scalars()
         )
         assert len(claim_rows) == 1
+        assert claim_rows[0].balance_snapshot == int(after_data["balance"])
+
+
+@pytest.mark.asyncio
+async def test_register_delete_reregister_restores_unspent_balance(
+    api_client: httpx.AsyncClient,
+    test_identity: IdentityData,
+) -> None:
+    email = str(test_identity["email"]).strip().lower()
+    bonus = int(config.points_policy.register_bonus_points)
+
+    first = await _create_email_session(
+        api_client,
+        email=email,
+        code=str(test_identity["code"]),
+    )
+    user1 = first.get("user")
+    assert isinstance(user1, dict)
+    user1_id = str(user1["id"])
+    token1 = str(first["access_token"])
+    headers1 = {"Authorization": f"Bearer {token1}"}
+
+    first_balance = await api_client.get("/api/v1/points/balance", headers=headers1)
+    first_balance.raise_for_status()
+    first_data = first_balance.json()
+    assert int(first_data["balance"]) == bonus
+
+    delete_resp = await api_client.delete("/api/v1/users/me", headers=headers1)
+    assert delete_resp.status_code == 204
+
+    second = await _create_email_session(
+        api_client,
+        email=email,
+        code=str(test_identity["code"]),
+    )
+    user2 = second.get("user")
+    assert isinstance(user2, dict)
+    user2_id = str(user2["id"])
+    assert user1_id != user2_id
+    token2 = str(second["access_token"])
+    headers2 = {"Authorization": f"Bearer {token2}"}
+
+    reregister_balance = await api_client.get(
+        "/api/v1/points/balance", headers=headers2
+    )
+    reregister_balance.raise_for_status()
+    re_data = reregister_balance.json()
+    assert int(re_data["balance"]) == bonus
+
+    cleanup_resp = await api_client.delete("/api/v1/users/me", headers=headers2)
+    assert cleanup_resp.status_code == 204