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/api/storefront.py

@@ -11,6 +11,7 @@ import logging
 from fastapi import APIRouter, Depends, Request
 from sqlalchemy.orm import Session
 
+from app.api.deps import require_platform
 from app.core.database import get_db
 from app.modules.cms.schemas import (
     ContentPageListItem,
@@ -29,7 +30,11 @@ logger = logging.getLogger(__name__)
 
 # public - storefront content pages are publicly accessible
 @router.get("/navigation", response_model=list[ContentPageListItem])
-def get_navigation_pages(request: Request, db: Session = Depends(get_db)):
+def get_navigation_pages(
+    request: Request,
+    platform=Depends(require_platform),
+    db: Session = Depends(get_db),
+):
     """
     Get list of content pages for navigation (footer/header).
 
@@ -37,9 +42,8 @@ def get_navigation_pages(request: Request, db: Session = Depends(get_db)):
     Returns store overrides + platform defaults.
     """
     store = getattr(request.state, "store", None)
-    platform = getattr(request.state, "platform", None)
     store_id = store.id if store else None
-    platform_id = platform.id if platform else 1
+    platform_id = platform.id
 
     # Get all published pages for this store
     pages = content_page_service.list_pages_for_store(
@@ -59,7 +63,12 @@ def get_navigation_pages(request: Request, db: Session = Depends(get_db)):
 
 
 @router.get("/{slug}", response_model=PublicContentPageResponse)
-def get_content_page(slug: str, request: Request, db: Session = Depends(get_db)):
+def get_content_page(
+    slug: str,
+    request: Request,
+    platform=Depends(require_platform),
+    db: Session = Depends(get_db),
+):
     """
     Get a specific content page by slug.
 
@@ -67,9 +76,8 @@ def get_content_page(slug: str, request: Request, db: Session = Depends(get_db))
     Returns store override if exists, otherwise platform default.
     """
     store = getattr(request.state, "store", None)
-    platform = getattr(request.state, "platform", None)
     store_id = store.id if store else None
-    platform_id = platform.id if platform else 1
+    platform_id = platform.id
 
     page = content_page_service.get_page_for_store_or_raise(
         db,