feat: platform-aware storefront routing and billing improvements

Overhaul storefront URL routing to be platform-aware:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (internally rewritten to /storefront/)
- Add subdomain detection in PlatformContextMiddleware
- Add /storefront/ path rewrite for prod mode (subdomain/custom domain)
- Remove all silent platform fallbacks (platform_id=1)
- Add require_platform dependency for clean endpoint validation
- Update route registration, templates, module definitions, base_url calc
- Update StoreContextMiddleware for /storefront/ path detection
- Remove /stores/ from FrontendDetector STOREFRONT_PATH_PREFIXES

Billing service improvements:
- Add store_platform_sync_service to keep store_platforms in sync
- Make tier lookups platform-aware across billing services
- Add tiers for all platforms in seed data
- Add demo subscriptions to seed

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

scripts/show_urls.py

@@ -138,15 +138,17 @@ def collect_dev_urls(platforms, stores, store_domains, store_platform_map):
             url = _store_dev_login_url(v.store_code)
             urls.append((f"Store Login: {v.name}", url, [200]))
 
-    # Storefronts
+    # Storefronts (platform-aware)
     for v in stores:
         if not v.is_active:
             continue
-        urls.append((
-            f"Storefront: {v.name}",
-            f"{DEV_BASE}/stores/{v.store_code}/storefront/",
-            [200, 302],
-        ))
+        platform_codes = store_platform_map.get(v.id, [])
+        for pc in platform_codes:
+            urls.append((
+                f"Storefront [{pc}]: {v.name}",
+                f"{DEV_BASE}/platforms/{pc}/storefront/{v.store_code}/",
+                [200, 302],
+            ))
 
     # Store info API (public, no auth needed)
     for v in stores:
@@ -225,7 +227,9 @@ def print_dev_urls(platforms, stores, store_domains, store_platform_map):
         tag = f" [{status_badge(v.is_active)}]" if not v.is_active else ""
         code = v.store_code
         print(f"      {v.name} ({code}){tag}")
-        print(f"        Shop:       {DEV_BASE}/stores/{code}/storefront/")
+        pcs = store_platform_map.get(v.id, [])
+        for pc in pcs:
+            print(f"        Shop [{pc}]:  {DEV_BASE}/platforms/{pc}/storefront/{code}/")
         print(f"        API:        {DEV_BASE}/api/v1/storefront/{code}/")