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/stripe_service.py

@@ -93,6 +93,38 @@ class StripeService:
         )
         return customer.id
 
+    def create_customer_for_merchant(
+        self,
+        merchant,
+        email: str,
+        name: str | None = None,
+        metadata: dict | None = None,
+    ) -> str:
+        """
+        Create a Stripe customer for a merchant (before store exists).
+
+        Used during signup when the store hasn't been created yet.
+        Returns the Stripe customer ID.
+        """
+        self._check_configured()
+
+        customer_metadata = {
+            "merchant_id": str(merchant.id),
+            "merchant_name": merchant.name,
+            **(metadata or {}),
+        }
+
+        customer = stripe.Customer.create(
+            email=email,
+            name=name or merchant.name,
+            metadata=customer_metadata,
+        )
+
+        logger.info(
+            f"Created Stripe customer {customer.id} for merchant {merchant.name}"
+        )
+        return customer.id
+
     def get_customer(self, customer_id: str) -> stripe.Customer:
         """Get a Stripe customer by ID."""
         self._check_configured()