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/cms/routes/pages/platform.py

@@ -91,8 +91,9 @@ async def homepage(
     if store:
         logger.debug(f"[HOMEPAGE] Store detected: {store.subdomain}")
 
-        # Get platform_id (use platform from context or default to 1 for OMS)
-        platform_id = platform.id if platform else 1
+        if not platform:
+            raise HTTPException(status_code=400, detail="Platform context required")
+        platform_id = platform.id
 
         # Try to find store landing page (slug='landing' or 'home')
         landing_page = content_page_service.get_page_for_store(
@@ -133,21 +134,19 @@ async def homepage(
             else "unknown"
         )
 
-        if access_method == "path":
-            full_prefix = (
-                store_context.get("full_prefix", "/store/")
-                if store_context
-                else "/store/"
-            )
+        if access_method == "path" and platform:
             return RedirectResponse(
-                url=f"{full_prefix}{store.subdomain}/storefront/", status_code=302
+                url=f"/platforms/{platform.code}/storefront/{store.store_code}/",
+                status_code=302,
             )
-        # Domain/subdomain - redirect to /storefront/
-        return RedirectResponse(url="/storefront/", status_code=302)
+        # Domain/subdomain - root is storefront
+        return RedirectResponse(url="/", status_code=302)
 
     # Scenario 2: Platform marketing site (no store)
     # Load platform homepage from CMS (slug='home')
-    platform_id = platform.id if platform else 1
+    if not platform:
+        raise HTTPException(status_code=400, detail="Platform context required")
+    platform_id = platform.id
 
     cms_homepage = content_page_service.get_platform_page(
         db, platform_id=platform_id, slug="home", include_unpublished=False
@@ -227,9 +226,10 @@ async def content_page(
     This is a catch-all route for dynamic content pages managed via the admin CMS.
     Platform pages have store_id=None and is_platform_page=True.
     """
-    # Get platform from middleware (default to OMS platform_id=1)
     platform = getattr(request.state, "platform", None)
-    platform_id = platform.id if platform else 1
+    if not platform:
+        raise HTTPException(status_code=400, detail="Platform context required")
+    platform_id = platform.id
 
     # Load platform marketing page from database
     page = content_page_service.get_platform_page(