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/contracts/base.py

@@ -0,0 +1,47 @@
+# app/modules/contracts/base.py
+"""
+Base protocol definitions for cross-module communication.
+
+Protocols define the interface that services must implement without
+requiring inheritance. This allows for:
+- Duck typing with static type checking
+- Easy mocking in tests
+- Module independence
+"""
+
+from typing import TYPE_CHECKING, Protocol, runtime_checkable
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session
+
+
+@runtime_checkable
+class ServiceProtocol(Protocol):
+    """
+    Base protocol for all module services.
+
+    Services should be stateless and operate on database sessions
+    passed as arguments. This ensures:
+    - Thread safety
+    - Transaction boundaries are clear
+    - Easy testing with mock sessions
+    """
+
+    pass
+
+
+@runtime_checkable
+class CRUDServiceProtocol(Protocol):
+    """
+    Protocol for services that provide CRUD operations.
+
+    All methods receive a database session as the first argument.
+    """
+
+    def get_by_id(self, db: "Session", id: int) -> object | None:
+        """Get entity by ID."""
+        ...
+
+    def delete(self, db: "Session", id: int) -> bool:
+        """Delete entity by ID. Returns True if deleted."""
+        ...