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_invites.dart

@@ -0,0 +1,32 @@
+import 'package:dio/dio.dart';
+
+import '../../models/invite.dart';
+
+class AdminInvitesApi {
+  AdminInvitesApi(this._dio);
+  final Dio _dio;
+
+  /// GET /api/admin/invites → `{"invites": [...]}`. Envelope unwrapped.
+  Future<List<Invite>> list() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/admin/invites');
+    final raw = (r.data?['invites'] as List?) ?? const [];
+    return raw
+        .map((e) => Invite.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+
+  /// POST /api/admin/invites — body is optional `{"note": "..."}`.
+  /// Server hardcodes the 24h TTL; admins cannot configure expiry.
+  /// Returns the bare invite (not enveloped).
+  Future<Invite> create({String? note}) async {
+    final r = await _dio.post<Map<String, dynamic>>(
+      '/api/admin/invites',
+      data: {if (note != null && note.isNotEmpty) 'note': note},
+    );
+    return Invite.fromJson(r.data ?? const {});
+  }
+
+  Future<void> revoke(String token) async {
+    await _dio.delete<void>('/api/admin/invites/$token');
+  }
+}