feat: implement self-contained module architecture (Phase 1 & 2)

Phase 1 - Foundation:
- Add app/modules/contracts/ with Protocol definitions for cross-module
  communication (ServiceProtocol, ContentServiceProtocol, MediaServiceProtocol)
- Enhance app/modules/base.py ModuleDefinition with self-contained module
  support (is_self_contained, services_path, models_path, etc.)
- Update app/templates_config.py with multi-directory template loading
  using Jinja2 ChoiceLoader for module templates

Phase 2 - CMS Pilot Module:
- Migrate CMS service to app/modules/cms/services/content_page_service.py
- Create app/modules/cms/exceptions.py with CMS-specific exceptions
- Configure app/modules/cms/models/ to re-export ContentPage from canonical
  location (models.database) to avoid circular imports
- Update cms_module definition with is_self_contained=True and paths
- Add backwards compatibility shims with deprecation warnings:
  - app/services/content_page_service.py -> app.modules.cms.services
  - app/exceptions/content_page.py -> app.modules.cms.exceptions

Note: SQLAlchemy models remain in models/database/ as the canonical location
to avoid circular imports at startup time. Module model packages re-export
from the canonical location.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/cms/__init__.py

@@ -2,8 +2,13 @@
 """
 CMS Module - Content Management System.
 
+This is a SELF-CONTAINED module that includes:
+- Services: content_page_service (business logic)
+- Models: ContentPage (database model)
+- Exceptions: ContentPageNotFoundException, etc.
+
 This module provides:
-- Content pages management
+- Content pages management (three-tier: platform, vendor default, vendor override)
 - Media library
 - Vendor themes
 - SEO tools
@@ -15,6 +20,16 @@ Routes:
 Menu Items:
 - Admin: content-pages, vendor-themes
 - Vendor: content-pages, media
+
+Usage:
+    # Preferred: Import from module directly
+    from app.modules.cms.services import content_page_service
+    from app.modules.cms.models import ContentPage
+    from app.modules.cms.exceptions import ContentPageNotFoundException
+
+    # Legacy: Still works via re-export shims (deprecated)
+    from app.services.content_page_service import content_page_service
+    from models.database.content_page import ContentPage
 """
 
 from app.modules.cms.definition import cms_module