feat(billing): B4-B5 — tests, lint fixes, CHANGELOG + PROJECT.md

- Fix unused imports in stripe.py (hashlib, hmac, time)
- Update test_billing_routes.py: insert into payment_products table,
  fix mock paths for extracted paddle.py, add Stripe webhook 404 test
- Update CHANGELOG.md with Stripe provider feature
- Update PROJECT.md Done section

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

web/src/padelnomics/billing/stripe.py

@@ -12,11 +12,8 @@ Exports the same interface as paddle.py so billing/routes.py can dispatch:
 Stripe Tax add-on handles EU VAT collection (must be enabled in Stripe Dashboard).
 """
 
-import hashlib
-import hmac
 import json
 import logging
-import time
 
 import stripe as stripe_sdk