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>

PROJECT.md

@@ -60,6 +60,7 @@
 - [x] Boost purchases (logo, highlight, verified, card color, sticky week/month)
 - [x] Credit pack purchases (25/50/100/250)
 - [x] Supplier subscription tiers (Basic free / Growth €199 / Pro €499, monthly + annual)
+- [x] **Stripe payment provider** — env-var toggle (`PAYMENT_PROVIDER=paddle|stripe`), Stripe Checkout Sessions + Billing Portal + webhook handling, `payment_products` table generalizes `paddle_products`, dual-path JS templates, `billing/paddle.py` + `billing/stripe.py` dispatch pattern, `setup_stripe.py` product creation script
 - [x] **Feature flags** (DB-backed, migration 0019) — `is_flag_enabled()` + `feature_gate()` decorator replace `WAITLIST_MODE`; 5 flags (markets, payments, planner_export, supplier_signup, lead_unlock); admin UI at `/admin/flags` with toggle
 - [x] **Pricing overhaul** — Basic free (no Paddle sub), card color €59, BP PDF €149; supplier page restructured value-first (why → guarantee → leads → social proof → pricing); all CTAs "Get Started Free"; static ROI line; credits-only callout
 - [x] **Lead-Back Guarantee** (migration 0020) — 1-click credit refund for non-responding leads (3–30 day window); `refund_lead_guarantee()` in credits.py; "Lead didn't respond" button on unlocked lead cards