test: add 18 e2e tests for billing, checkout, supplier signup/dashboard, export

- Pricing page (EN/DE, plan cards, no-auth access)
- Checkout success (auth required, renders for authed user)
- Supplier signup wizard (step 1, plan cards, DE variant, success page)
- Supplier dashboard (overview stats, boosts/credit packs, listing, leads tabs)
- Business plan export (auth required, form renders)

Also fixes:
- E2e server init_db mock scope — before_serving was calling real init_db
  outside the patch context, overwriting the in-memory DB (fixes 3
  pre-existing failures: markets_hub, markets_results, signup_page)
- Add _seed_billing_data() for supplier + feature flags in e2e server
- Mock RESEND_API_KEY="" in conftest + e2e server to prevent real emails

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

web/tests/conftest.py

@@ -189,6 +189,7 @@ def patch_config():
     test_values = {
         "PADDLE_API_KEY": "test_api_key_123",
         "PADDLE_WEBHOOK_SECRET": "whsec_test_secret",
+        "RESEND_API_KEY": "",  # never send real emails in tests
         "BASE_URL": "http://localhost:5000",
         "DEBUG": True,
         "WAITLIST_MODE": False,