feat: add CMS service layer and API endpoints

Implement complete CMS business logic and REST API:

Service Layer (content_page_service.py):
- get_page_for_vendor() - Two-tier lookup with fallback
- list_pages_for_vendor() - Merge vendor + platform pages
- create_page(), update_page(), delete_page() - CRUD operations
- Support for published/draft workflow
- Footer/header navigation filtering

API Endpoints:

Admin API (/api/v1/admin/content-pages):
- POST /platform - Create platform defaults
- GET /platform - List platform defaults
- GET / - List all pages with vendor filtering
- PUT /{id} - Update any page
- DELETE /{id} - Delete any page

Vendor API (/api/v1/vendor/{code}/content-pages):
- GET / - List available pages (vendor + platform merged)
- GET /overrides - List only vendor overrides
- POST / - Create vendor override
- PUT /{id} - Update vendor page
- DELETE /{id} - Delete vendor page

Shop API (/api/v1/shop/content-pages):
- GET /navigation - Get footer/header navigation pages
- GET /{slug} - Get specific page (public, with fallback)

All endpoints include proper authentication, authorization,
and validation using Pydantic schemas.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/api/v1/vendor/__init__.py

@@ -29,6 +29,7 @@ from . import (
     media,
     notifications,
     analytics,
+    content_pages,
 )
 
 
@@ -66,6 +67,9 @@ router.include_router(media.router, tags=["vendor-media"])
 router.include_router(notifications.router, tags=["vendor-notifications"])
 router.include_router(analytics.router, tags=["vendor-analytics"])
 
+# Content pages management
+router.include_router(content_pages.router, prefix="/{vendor_code}/content-pages", tags=["vendor-content-pages"])
+
 # Vendor info endpoint - MUST BE LAST! Has catch-all GET /{vendor_code}
 router.include_router(info.router, tags=["vendor-info"])