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/models/__init__.py

@@ -0,0 +1,20 @@
+# app/modules/cms/models/__init__.py
+"""
+CMS module database models.
+
+This package re-exports the ContentPage model from its canonical location
+in models.database. SQLAlchemy models must remain in a single location to
+avoid circular imports at startup time.
+
+Usage:
+    from app.modules.cms.models import ContentPage
+
+The canonical model is at: models.database.content_page.ContentPage
+"""
+
+# Import from canonical location to avoid circular imports
+from models.database.content_page import ContentPage
+
+__all__ = [
+    "ContentPage",
+]