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/definition.py

@@ -3,7 +3,15 @@
 CMS module definition.
 
 Defines the CMS module including its features, menu items,
-and route configurations.
+route configurations, and self-contained component paths.
+
+This is a self-contained module with:
+- Services: app.modules.cms.services
+- Models: app.modules.cms.models
+- Exceptions: app.modules.cms.exceptions
+
+Templates remain in core (app/templates/admin/) for now due to
+admin/base.html inheritance dependency.
 """
 
 from app.modules.base import ModuleDefinition
@@ -24,7 +32,7 @@ def _get_vendor_router():
     return vendor_router
 
 
-# CMS module definition
+# CMS module definition - Self-contained module (pilot)
 cms_module = ModuleDefinition(
     code="cms",
     name="Content Management",
@@ -48,6 +56,13 @@ cms_module = ModuleDefinition(
         ],
     },
     is_core=False,
+    # Self-contained module configuration
+    is_self_contained=True,
+    services_path="app.modules.cms.services",
+    models_path="app.modules.cms.models",
+    exceptions_path="app.modules.cms.exceptions",
+    # Templates remain in core for now (admin/content-pages*.html)
+    templates_path=None,
 )