feat(billing): end-to-end Stripe subscription signup with platform enforcement

Move core signup service from marketplace to billing module, add
automatic Stripe product/price sync for tiers, create loyalty-specific
signup wizard, and enforce that platform is always explicitly known
(no silent defaulting to primary/hardcoded ID).

Key changes:
- New billing SignupService with separated account/store creation steps
- Stripe auto-sync on tier create/update (new prices, archive old)
- Loyalty signup template (Plan → Account → Store → Payment)
- platform_code is now required throughout the signup flow
- Pricing/signup pages return 404 if platform not detected
- OMS-specific logic (Letzshop claiming) stays in marketplace module
- Bootstrap script: scripts/seed/sync_stripe_products.py

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

app/modules/marketplace/tests/unit/test_platform_signup_service.py

@@ -1,19 +1,17 @@
-"""Unit tests for PlatformSignupService."""
+"""Unit tests for PlatformSignupService (now in billing module)."""
 
 import pytest
 
-from app.modules.marketplace.services.platform_signup_service import (
-    PlatformSignupService,
-)
+from app.modules.billing.services.signup_service import SignupService
 
 
 @pytest.mark.unit
 @pytest.mark.marketplace
 class TestPlatformSignupService:
-    """Test suite for PlatformSignupService."""
+    """Test suite for SignupService (moved from marketplace to billing)."""
 
     def setup_method(self):
-        self.service = PlatformSignupService()
+        self.service = SignupService()
 
     def test_service_instantiation(self):
         """Service can be instantiated."""