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)
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