fix(billing): make Stripe webhook idempotency atomic so failed handlers can retry

Previously `apply_subscription_event` committed the StripeEvent idempotency
row before invoking the handler, then the handlers each committed their own
mutations. If a handler raised mid-flight (transient DB error, network blip,
race), the idempotency mark was already persisted — Stripe's retry would hit
the IntegrityError branch and silently return False, and the subscription
state would permanently desync from Stripe.

Switch to a single atomic transaction:
- Insert the StripeEvent + flush (catch IntegrityError on duplicate event_id).
- Run the handler.
- Commit on success; roll back the entire transaction on failure and re-raise.

Drop the four `db.commit()` calls inside `_handle_*` so the outer caller owns
commit. The webhook endpoint already lets exceptions propagate, so a 500
response now correctly tells Stripe to retry.

Tests: three new regression cases in test_stripe_webhook_handler.py covering
handler failure (no idempotency mark persisted), retry-after-failure success,
and duplicate-event-id skip.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_stripe_webhook_handler.py

@@ -142,3 +142,178 @@ async def test_webhook_idempotency(
     assert r2.status_code == 200
     assert r1.json()["applied"] is True
     assert r2.json()["applied"] is False
+
+
+# ----------------------------------------------------------------------------
+# Atomic-idempotency regression tests
+# ----------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_apply_event_handler_failure_does_not_persist_idempotency_mark(
+    test_db, test_user,
+):
+    """If the handler raises, the StripeEvent row must NOT be persisted —
+    otherwise Stripe's retry will be silently dropped as a duplicate and the
+    subscription state will desync from Stripe."""
+    from app.services.billing import BillingService
+    from app.models.stripe_event import StripeEvent
+
+    event_id = "evt_handler_fail_1"
+    payload = {"data": {"object": {
+        "id": "sub_doesnotmatter",
+        "status": "active",
+        "current_period_start": 1714521600,
+        "current_period_end": 1717113600,
+        "items": {"data": [{"quantity": 1}]},
+        "cancel_at_period_end": False,
+    }}}
+
+    boom = RuntimeError("simulated handler failure")
+    with patch(
+        "app.services.billing._handle_subscription_updated",
+        side_effect=boom,
+    ):
+        with pytest.raises(RuntimeError, match="simulated handler failure"):
+            await BillingService.apply_subscription_event(
+                test_db,
+                event_id=event_id,
+                event_type="customer.subscription.updated",
+                payload=payload,
+            )
+
+    # The StripeEvent row must not exist — handler raised, the entire
+    # transaction (idempotency mark + partial mutations) was rolled back.
+    row = (await test_db.execute(
+        select(StripeEvent).where(StripeEvent.id == event_id)
+    )).scalar_one_or_none()
+    assert row is None, (
+        "StripeEvent row was persisted despite handler failure — "
+        "Stripe retry will be silently dropped"
+    )
+
+
+@pytest.mark.asyncio
+async def test_apply_event_retry_after_failure_succeeds(
+    test_db, test_user,
+):
+    """A failed first attempt followed by a successful retry must apply state.
+    This is the core Stripe webhook retry contract."""
+    from app.services.billing import BillingService
+    from app.models.stripe_event import StripeEvent
+
+    account_id = uuid.UUID(test_user["user_data"]["account_id"])
+    await test_db.execute(delete(Subscription).where(Subscription.account_id == account_id))
+    test_db.add(Subscription(
+        account_id=account_id, plan="pro", status="trialing",
+        stripe_subscription_id="sub_retry",
+    ))
+    await test_db.commit()
+
+    event_id = "evt_retry_1"
+    payload = {"data": {"object": {
+        "id": "sub_retry",
+        "status": "active",
+        "current_period_start": 1714521600,
+        "current_period_end": 1717113600,
+        "items": {"data": [{"quantity": 3}]},
+        "cancel_at_period_end": False,
+    }}}
+
+    # First attempt — handler raises mid-flight.
+    with patch(
+        "app.services.billing._handle_subscription_updated",
+        side_effect=RuntimeError("transient blip"),
+    ):
+        with pytest.raises(RuntimeError):
+            await BillingService.apply_subscription_event(
+                test_db,
+                event_id=event_id,
+                event_type="customer.subscription.updated",
+                payload=payload,
+            )
+
+    # No idempotency mark, sub still trialing.
+    row = (await test_db.execute(
+        select(StripeEvent).where(StripeEvent.id == event_id)
+    )).scalar_one_or_none()
+    assert row is None
+    sub = (await test_db.execute(
+        select(Subscription).where(Subscription.account_id == account_id)
+    )).scalar_one()
+    assert sub.status == "trialing"
+
+    # Second attempt — same event_id, handler succeeds.
+    applied = await BillingService.apply_subscription_event(
+        test_db,
+        event_id=event_id,
+        event_type="customer.subscription.updated",
+        payload=payload,
+    )
+    assert applied is True
+
+    # Idempotency mark now persisted, sub state reconciled.
+    row = (await test_db.execute(
+        select(StripeEvent).where(StripeEvent.id == event_id)
+    )).scalar_one()
+    assert row.id == event_id
+    await test_db.refresh(sub)
+    assert sub.status == "active"
+    assert sub.seat_limit == 3
+
+
+@pytest.mark.asyncio
+async def test_apply_event_duplicate_event_id_skips(
+    test_db, test_user,
+):
+    """Two successful invocations with the same event_id must not double-apply.
+    Second call returns False; mutations are not repeated."""
+    from app.services.billing import BillingService
+
+    account_id = uuid.UUID(test_user["user_data"]["account_id"])
+    await test_db.execute(delete(Subscription).where(Subscription.account_id == account_id))
+    test_db.add(Subscription(
+        account_id=account_id, plan="pro", status="trialing",
+        stripe_subscription_id="sub_dup",
+    ))
+    await test_db.commit()
+
+    event_id = "evt_dedupe_1"
+    payload = {"data": {"object": {
+        "id": "sub_dup",
+        "status": "active",
+        "current_period_start": 1714521600,
+        "current_period_end": 1717113600,
+        "items": {"data": [{"quantity": 7}]},
+        "cancel_at_period_end": False,
+    }}}
+
+    applied1 = await BillingService.apply_subscription_event(
+        test_db,
+        event_id=event_id,
+        event_type="customer.subscription.updated",
+        payload=payload,
+    )
+    assert applied1 is True
+
+    sub = (await test_db.execute(
+        select(Subscription).where(Subscription.account_id == account_id)
+    )).scalar_one()
+    assert sub.status == "active"
+    assert sub.seat_limit == 7
+
+    # Mutate locally so we can prove the second call doesn't re-run the handler.
+    sub.seat_limit = 99
+    await test_db.commit()
+
+    applied2 = await BillingService.apply_subscription_event(
+        test_db,
+        event_id=event_id,
+        event_type="customer.subscription.updated",
+        payload=payload,
+    )
+    assert applied2 is False
+
+    await test_db.refresh(sub)
+    # Handler did NOT run again — our local mutation is preserved.
+    assert sub.seat_limit == 99