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>

scripts/seed/sync_stripe_products.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+"""
+Sync Subscription Tiers to Stripe.
+
+Creates Stripe Products and Prices for all subscription tiers that don't
+have them yet. Safe to run multiple times (idempotent).
+
+Usage:
+    python scripts/seed/sync_stripe_products.py
+"""
+
+import sys
+from pathlib import Path
+
+# Add project root to path
+project_root = Path(__file__).parent.parent
+sys.path.insert(0, str(project_root))
+
+from app.core.config import settings
+from app.core.database import SessionLocal
+from app.modules.billing.models.subscription import SubscriptionTier
+from app.modules.billing.services.admin_subscription_service import (
+    AdminSubscriptionService,
+)
+
+
+def main():
+    if not settings.stripe_secret_key:
+        print("ERROR: STRIPE_SECRET_KEY not configured. Set it in .env")
+        sys.exit(1)
+
+    db = SessionLocal()
+    try:
+        tiers = db.query(SubscriptionTier).order_by(
+            SubscriptionTier.platform_id,
+            SubscriptionTier.display_order,
+        ).all()
+
+        if not tiers:
+            print("No subscription tiers found in database.")
+            return
+
+        print(f"Found {len(tiers)} tiers to sync.\n")
+
+        synced = 0
+        skipped = 0
+        for tier in tiers:
+            already_synced = (
+                tier.stripe_product_id
+                and tier.stripe_price_monthly_id
+                and (tier.stripe_price_annual_id or not tier.price_annual_cents)
+            )
+
+            print(f"  [{tier.code}] {tier.name} (platform_id={tier.platform_id})")
+            if already_synced:
+                print(f"    -> Already synced (product={tier.stripe_product_id})")
+                skipped += 1
+                continue
+
+            AdminSubscriptionService._sync_tier_to_stripe(db, tier)
+            db.commit()
+            synced += 1
+            print(f"    -> Product: {tier.stripe_product_id}")
+            print(f"    -> Monthly: {tier.stripe_price_monthly_id}")
+            if tier.stripe_price_annual_id:
+                print(f"    -> Annual:  {tier.stripe_price_annual_id}")
+
+        print(f"\nDone. Synced: {synced}, Already up-to-date: {skipped}")
+
+    finally:
+        db.close()
+
+
+if __name__ == "__main__":
+    main()