feat: add DELETE /api/chat/:caseId to clear a case's chat history

Frontend's chat history trash icon was calling this endpoint but it
didn't exist yet, causing a JSON-parse error on the default 404 page.

Also renames the LegalOS health-check message to AyrisLegal.

src/routes/chat.routes.ts

@@ -1,9 +1,10 @@
 import { Router } from 'express';
 import { requireAuth } from '../middleware/auth';
-import { chatMessage } from '../controllers/chat.controller';
+import { chatMessage, deleteChatHistory } from '../controllers/chat.controller';
 
 const router = Router();
 
 router.post('/:caseId/message', requireAuth, chatMessage);
+router.delete('/:caseId', requireAuth, deleteChatHistory);
 
 export default router;