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/index.ts

@@ -6,7 +6,7 @@ import precedentRoutes from './precedent.routes';
 const router = Router();
 
 router.get('/health', (req, res) => {
-  res.json({ status: 'ok', message: 'LegalOS Backend is running' });
+  res.json({ status: 'ok', message: 'AyrisLegal Backend is running' });
 });
 
 router.use('/documents', documentRoutes);