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/admin/__init__.py

@@ -34,7 +34,8 @@ from . import (
     monitoring,
     audit,
     settings,
-    notifications
+    notifications,
+    content_pages
 )
 
 # Create admin router
@@ -62,6 +63,9 @@ router.include_router(vendor_domains.router, tags=["admin-vendor-domains"])
 # Include vendor themes management endpoints
 router.include_router(vendor_themes.router, tags=["admin-vendor-themes"])
 
+# Include content pages management endpoints
+router.include_router(content_pages.router, prefix="/content-pages", tags=["admin-content-pages"])
+
 
 # ============================================================================
 # User Management