feat(flutter/api): admin endpoint clients

Four small Dio-backed API classes mirroring the server's actual
contract (verified against internal/api/admin_*.go):

- Requests/Quarantine return flat lists; Users/Invites are enveloped
  ({"users": [...]}, {"invites": [...]}) and unwrap here
- setAutoApprove sends {auto_approve: bool} — different from the
  response field name auto_approve_requests
- resetPassword takes admin-supplied {password: string}, returns 204
- Invite create takes optional {note: string}; expiry is server-fixed
  at 24h

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/api/endpoints/admin_users.dart

@@ -0,0 +1,47 @@
+import 'package:dio/dio.dart';
+
+import '../../models/admin_user.dart';
+
+class AdminUsersApi {
+  AdminUsersApi(this._dio);
+  final Dio _dio;
+
+  /// GET /api/admin/users → `{"users": [...]}`. Envelope unwrapped here.
+  Future<List<AdminUser>> list() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/admin/users');
+    final raw = (r.data?['users'] as List?) ?? const [];
+    return raw
+        .map((e) => AdminUser.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+
+  Future<void> setAdmin(String id, bool isAdmin) async {
+    await _dio.put<void>(
+      '/api/admin/users/$id/admin',
+      data: {'is_admin': isAdmin},
+    );
+  }
+
+  /// Server's body field is `auto_approve` (NOT `auto_approve_requests`)
+  /// — different from the response field name. Verified against
+  /// internal/api/admin_users.go `adminAutoApproveReq` (May 2026).
+  Future<void> setAutoApprove(String id, bool autoApprove) async {
+    await _dio.put<void>(
+      '/api/admin/users/$id/auto-approve',
+      data: {'auto_approve': autoApprove},
+    );
+  }
+
+  /// Admin supplies the new password; server returns 204. There is no
+  /// server-generated-password mode.
+  Future<void> resetPassword(String id, String newPassword) async {
+    await _dio.post<void>(
+      '/api/admin/users/$id/reset-password',
+      data: {'password': newPassword},
+    );
+  }
+
+  Future<void> delete(String id) async {
+    await _dio.delete<void>('/api/admin/users/$id');
+  }
+}