refactor: migrate messaging and media routes to modules

Messaging module (communication):
- vendor_messages.py: Conversation and message management
- vendor_notifications.py: Vendor notifications
- vendor_email_settings.py: SMTP and provider configuration
- vendor_email_templates.py: Email template customization

CMS module (content management):
- vendor_media.py: Media library management
- vendor_content_pages.py: Content page overrides

All routes auto-discovered via is_self_contained=True.
Deleted 5 legacy files from app/api/v1/vendor/.
app/api/v1/vendor/ now empty except for __init__.py (auto-discovery only).

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

app/modules/messaging/routes/api/__init__.py

@@ -1,9 +1,21 @@
 # app/modules/messaging/routes/api/__init__.py
-"""Messaging module API routes."""
+"""
+Messaging module API routes.
+
+Vendor routes:
+- /messages/* - Conversation and message management
+- /notifications/* - Vendor notifications
+- /email-settings/* - SMTP and provider configuration
+- /email-templates/* - Email template customization
+
+Storefront routes:
+- Customer-facing messaging
+"""
 
 from app.modules.messaging.routes.api.storefront import router as storefront_router
+from app.modules.messaging.routes.api.vendor import vendor_router
 
 # Tag for OpenAPI documentation
 STOREFRONT_TAG = "Messages (Storefront)"
 
-__all__ = ["storefront_router", "STOREFRONT_TAG"]
+__all__ = ["storefront_router", "vendor_router", "STOREFRONT_TAG"]