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/config.py

@@ -31,6 +31,12 @@ class Settings(BaseSettings):
     # Uptime monitoring
     monitor_check_interval: int = 60
 
+    # Stripe Checkout Links
+    stripe_pro_checkout_url: str = ""       # e.g. https://buy.stripe.com/xxxx_pro
+    stripe_team_checkout_url: str = ""      # e.g. https://buy.stripe.com/xxxx_team
+    stripe_webhook_secret: str = ""         # e.g. whsec_xxxx
+    stripe_api_key: str = ""                # e.g. sk_test_xxxx (needed for webhook verification)
+
     model_config = {"env_file": ".env", "env_file_encoding": "utf-8"}
 
     @property