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/core/utils/page_context.py

@@ -332,14 +332,21 @@ def get_storefront_context(
     )
 
     # Calculate base URL for links
+    # Dev path-based: /platforms/{code}/storefront/{store_code}/
+    # Prod subdomain/custom domain: /
     base_url = "/"
     if access_method == "path" and store:
-        full_prefix = (
-            store_context.get("full_prefix", "/store/")
-            if store_context
-            else "/store/"
-        )
-        base_url = f"{full_prefix}{store.subdomain}/"
+        platform = getattr(request.state, "platform", None)
+        platform_original_path = getattr(request.state, "platform_original_path", None)
+        if platform and platform_original_path and platform_original_path.startswith("/platforms/"):
+            base_url = f"/platforms/{platform.code}/storefront/{store.store_code}/"
+        else:
+            full_prefix = (
+                store_context.get("full_prefix", "/storefront/")
+                if store_context
+                else "/storefront/"
+            )
+            base_url = f"{full_prefix}{store.store_code}/"
 
     # Read subscription info set by StorefrontAccessMiddleware
     subscription = getattr(request.state, "subscription", None)