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/templates_config.py

@@ -4,10 +4,21 @@ Shared Jinja2 templates configuration.
 
 All route modules should import `templates` from here to ensure
 consistent globals (like translation function) are available.
+
+Template Loading Strategy:
+- Core templates from app/templates/ (highest priority)
+- Module templates from app/modules/<module>/templates/ (namespaced)
+
+Module templates should use namespace prefix to avoid collisions:
+    app/modules/cms/templates/cms/admin/pages.html
+    -> Rendered as: templates.TemplateResponse("cms/admin/pages.html", ...)
 """
+
+import logging
 from pathlib import Path
 
 from fastapi.templating import Jinja2Templates
+from jinja2 import ChoiceLoader, FileSystemLoader
 
 from app.utils.i18n import (
     LANGUAGE_FLAGS,
@@ -17,11 +28,60 @@ from app.utils.i18n import (
     create_translation_context,
 )
 
-# Templates directory
-TEMPLATES_DIR = Path(__file__).parent / "templates"
+logger = logging.getLogger(__name__)
 
-# Create shared templates instance
+# Core templates directory
+TEMPLATES_DIR = Path(__file__).parent / "templates"
+MODULES_DIR = Path(__file__).parent / "modules"
+
+
+def create_template_loaders() -> ChoiceLoader:
+    """
+    Create a ChoiceLoader that searches multiple template directories.
+
+    Search order:
+    1. Core templates (app/templates/) - highest priority
+    2. Module templates (app/modules/<module>/templates/) - in alphabetical order
+
+    Returns:
+        ChoiceLoader configured with all template directories
+    """
+    loaders = [FileSystemLoader(str(TEMPLATES_DIR))]  # Core templates first
+
+    # Add module template directories
+    if MODULES_DIR.exists():
+        for module_dir in sorted(MODULES_DIR.iterdir()):
+            if module_dir.is_dir():
+                templates_path = module_dir / "templates"
+                if templates_path.exists() and templates_path.is_dir():
+                    loaders.append(FileSystemLoader(str(templates_path)))
+                    logger.debug(f"[Templates] Added module templates: {module_dir.name}")
+
+    return ChoiceLoader(loaders)
+
+
+def get_module_template_dirs() -> list[Path]:
+    """
+    Get list of all module template directories.
+
+    Useful for debugging and introspection.
+
+    Returns:
+        List of Path objects for module template directories
+    """
+    dirs = []
+    if MODULES_DIR.exists():
+        for module_dir in sorted(MODULES_DIR.iterdir()):
+            if module_dir.is_dir():
+                templates_path = module_dir / "templates"
+                if templates_path.exists() and templates_path.is_dir():
+                    dirs.append(templates_path)
+    return dirs
+
+
+# Create shared templates instance with multi-directory loader
 templates = Jinja2Templates(directory=str(TEMPLATES_DIR))
+templates.env.loader = create_template_loaders()
 
 # Add translation function to Jinja2 environment globals
 # This makes _() available in all templates AND macros