feat(billing): apply_subscription_event with stripe_events idempotency

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

backend/tests/test_billing_service.py

@@ -57,3 +57,24 @@ async def test_register_creates_trial_subscription(client, test_db):
     assert sub.plan == "pro"
     assert sub.status == "trialing"
     assert sub.current_period_end is not None
+
+
+@pytest.mark.asyncio
+async def test_apply_subscription_event_is_idempotent(test_db):
+    payload = {
+        "data": {"object": {
+            "id": "evt_test_1",
+            "customer": "cus_xxx",
+            "subscription": "sub_xxx",
+            "status": "active",
+        }}
+    }
+
+    applied_first = await BillingService.apply_subscription_event(
+        test_db, "evt_test_1", "customer.subscription.updated", payload
+    )
+    applied_second = await BillingService.apply_subscription_event(
+        test_db, "evt_test_1", "customer.subscription.updated", payload
+    )
+    assert applied_first is True
+    assert applied_second is False  # already-processed → ack without re-applying