feat(auth): add GET /keys to list available key identifiers

src/auth/auth.service.ts

@@ -40,6 +40,13 @@ export class AuthService {
     );
   }
 
+  listKeys(): string[] {
+    if (!fs.existsSync(this.keysDir)) return [];
+    return fs.readdirSync(this.keysDir)
+      .filter(f => f.endsWith('.json'))
+      .map(f => path.basename(f, '.json'));
+  }
+
   async login(keyId: string, sessionName?: string): Promise<{ token: string; sessionName: string }> {
     const resolvedSession = sessionName ?? crypto.randomUUID();