feat: add vendor messaging interface

- Add vendor messages API endpoints (/api/v1/vendor/messages)
- Add vendor messages page route (/vendor/{code}/messages)
- Add messages.html template for vendor portal
- Add messages.js Alpine component
- Add Messages link to vendor sidebar under Sales section

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/vendor/__init__.py

@@ -24,6 +24,7 @@ from . import (
     letzshop,
     marketplace,
     media,
+    messages,
     notifications,
     order_item_exceptions,
     orders,
@@ -68,6 +69,7 @@ router.include_router(letzshop.router, tags=["vendor-letzshop"])
 router.include_router(payments.router, tags=["vendor-payments"])
 router.include_router(media.router, tags=["vendor-media"])
 router.include_router(notifications.router, tags=["vendor-notifications"])
+router.include_router(messages.router, tags=["vendor-messages"])
 router.include_router(analytics.router, tags=["vendor-analytics"])
 
 # Content pages management