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/billing/services/__init__.py

@@ -21,6 +21,10 @@ from app.modules.billing.services.platform_pricing_service import (
     PlatformPricingService,
     platform_pricing_service,
 )
+from app.modules.billing.services.signup_service import (
+    SignupService,
+    signup_service,
+)
 from app.modules.billing.services.store_platform_sync_service import (
     StorePlatformSync,
     store_platform_sync,
@@ -65,4 +69,6 @@ __all__ = [
     "TierInfoData",
     "UpgradeTierData",
     "LimitCheckData",
+    "SignupService",
+    "signup_service",
 ]