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)

.env.example

@@ -24,4 +24,11 @@ SMTP_FROM=noreply@example.com
 WEBHOOK_NOTIFY_URL=
 
 # Uptime Monitoring
-MONITOR_CHECK_INTERVAL=60
+MONITOR_CHECK_INTERVAL=60
+
+# Stripe Checkout Links (set these in production)
+# Create Payment Links in Stripe Dashboard → Products → Payment Links
+STRIPE_PRO_CHECKOUT_URL=
+STRIPE_TEAM_CHECKOUT_URL=
+STRIPE_WEBHOOK_SECRET=
+STRIPE_API_KEY=