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>

app/modules/customers/definition.py

@@ -142,28 +142,28 @@ customers_module = ModuleDefinition(
                         id="dashboard",
                         label_key="storefront.account.dashboard",
                         icon="home",
-                        route="storefront/account/dashboard",
+                        route="account/dashboard",
                         order=10,
                     ),
                     MenuItemDefinition(
                         id="profile",
                         label_key="storefront.account.profile",
                         icon="user",
-                        route="storefront/account/profile",
+                        route="account/profile",
                         order=20,
                     ),
                     MenuItemDefinition(
                         id="addresses",
                         label_key="storefront.account.addresses",
                         icon="map-pin",
-                        route="storefront/account/addresses",
+                        route="account/addresses",
                         order=30,
                     ),
                     MenuItemDefinition(
                         id="settings",
                         label_key="storefront.account.settings",
                         icon="cog",
-                        route="storefront/account/settings",
+                        route="account/settings",
                         order=90,
                     ),
                 ],