feat: add case creation endpoint and auto-title from first message

POST /api/cases lets the frontend actually create a new dava dosyası
(no working "Yeni Dosya" flow existed before). chatMessage now
generates a short title from a case's first message (like Claude/
ChatGPT auto-titling) and renames the case, returned as `newTitle`.

src/routes/case.routes.ts

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