fix(billing): handle customer.subscription.created webhook + test isolation

- Add customer.subscription.created → subscription.activated mapping in
  stripe.parse_webhook so direct API subscription creation also creates DB rows
- Add customer.subscription.created to setup_stripe.py enabled_events
- Pin PAYMENT_PROVIDER=paddle and STRIPE_WEBHOOK_SECRET="" in test conftest
  so billing tests don't hit real Stripe API when env has Stripe keys
- Add 8 unit tests for stripe.parse_webhook covering all event types

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/tests/conftest.py

@@ -213,8 +213,10 @@ def patch_config():
     """Set test Paddle config values."""
     original_values = {}
     test_values = {
+        "PAYMENT_PROVIDER": "paddle",  # default to Paddle so mocks work
         "PADDLE_API_KEY": "test_api_key_123",
         "PADDLE_WEBHOOK_SECRET": "whsec_test_secret",
+        "STRIPE_WEBHOOK_SECRET": "",  # no Stripe in default tests
         "RESEND_API_KEY": "",  # never send real emails in tests
         "BASE_URL": "http://localhost:5000",
         "DEBUG": True,