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)

pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
     "passlib[bcrypt]>=1.7,<2.0",
     "bcrypt==4.0.1",
     "email-validator>=2.0,<3.0",
+    "stripe>=5.0,<16.0",
 ]
 
 [project.optional-dependencies]