feat: add DELETE /api/cases/:caseId to fully delete a case

Chat panel's trash icon should remove the whole case (not just clear
its chat history) — documents/analyses/chat_messages cascade-delete
via the existing ON DELETE CASCADE foreign keys.

src/routes/case.routes.ts

@@ -0,0 +1,9 @@
+import { Router } from 'express';
+import { requireAuth } from '../middleware/auth';
+import { deleteCase } from '../controllers/case.controller';
+
+const router = Router();
+
+router.delete('/:caseId', requireAuth, deleteCase);
+
+export default router;