feat: Stripe Checkout Link integration — billing API with 29 tests

- Add billing module (app/api/billing.py) with 5 API endpoints:
  - GET /api/v1/billing/checkout/{tier} — redirect to Stripe Payment Links
  - GET /api/v1/billing/status — current org tier, limits, upgrade URLs
  - GET /api/v1/billing/success — Stripe success callback
  - GET /api/v1/billing/cancel — Stripe cancel callback
  - POST /api/v1/billing/webhook — handles 5 Stripe event types

- Zero-code payment flow: uses pre-configured Stripe Payment Links
  with client_reference_id (org ID) and prefilled_email params

- Webhook handler processes checkout.session.completed,
  customer.subscription.updated/deleted, invoice events

- Stripe signature verification via stripe library (primary)
  or manual HMAC-SHA256 (fallback)

- Tier determination from payment amount: =pro, 9=team

- 4 new config settings in app/config.py:
  stripe_pro_checkout_url, stripe_team_checkout_url,
  stripe_webhook_secret, stripe_api_key

- Added stripe>=5.0,<16.0 dependency

- 29 tests in tests/test_billing.py (all passing)
- Total: 98 tests passing (69 existing + 29 new)

app/api/router.py

@@ -6,12 +6,14 @@ from app.api.monitors import router as monitors_router
 from app.api.subscribers import router as subscribers_router
 from app.api.settings import router as settings_router
 from app.api.organizations import router as organizations_router
+from app.api.billing import router as billing_router
 from app.routes.auth import router as auth_router
 
 api_v1_router = APIRouter()
 
 api_v1_router.include_router(auth_router, tags=["auth"])
 api_v1_router.include_router(organizations_router, prefix="/organizations", tags=["organizations"])
+api_v1_router.include_router(billing_router, prefix="/billing", tags=["billing"])
 api_v1_router.include_router(services_router, prefix="/services", tags=["services"])
 api_v1_router.include_router(incidents_router, prefix="/incidents", tags=["incidents"])
 api_v1_router.include_router(monitors_router, prefix="/monitors", tags=["monitors"])