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/api/deps.py

@@ -155,6 +155,23 @@ def _get_user_model(user_context: UserContext, db: Session) -> UserModel:
     return user
 
 
+# ============================================================================
+# PLATFORM CONTEXT
+# ============================================================================
+
+
+def require_platform(request: Request):
+    """Dependency that requires platform context from middleware.
+
+    Raises HTTPException(400) if no platform is set on request.state.
+    Use as a FastAPI dependency in endpoints that need platform context.
+    """
+    platform = getattr(request.state, "platform", None)
+    if not platform:
+        raise HTTPException(status_code=400, detail="Platform context required")
+    return platform
+
+
 # ============================================================================
 # ADMIN AUTHENTICATION
 # ============================================================================