feat(flutter/admin): API providers + adminCountsProvider

Single landing-screen provider fans out to requests/quarantine/users
list endpoints in parallel and returns counts for the section-card
badges. Per-section AsyncNotifier controllers land alongside their
screens in subsequent commits.

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

flutter_client/lib/admin/admin_providers.dart

@@ -0,0 +1,53 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../api/endpoints/admin_invites.dart';
+import '../api/endpoints/admin_quarantine.dart';
+import '../api/endpoints/admin_requests.dart';
+import '../api/endpoints/admin_users.dart';
+import '../library/library_providers.dart' show dioProvider;
+
+final adminRequestsApiProvider = FutureProvider<AdminRequestsApi>((ref) async {
+  return AdminRequestsApi(await ref.watch(dioProvider.future));
+});
+
+final adminQuarantineApiProvider =
+    FutureProvider<AdminQuarantineApi>((ref) async {
+  return AdminQuarantineApi(await ref.watch(dioProvider.future));
+});
+
+final adminUsersApiProvider = FutureProvider<AdminUsersApi>((ref) async {
+  return AdminUsersApi(await ref.watch(dioProvider.future));
+});
+
+final adminInvitesApiProvider = FutureProvider<AdminInvitesApi>((ref) async {
+  return AdminInvitesApi(await ref.watch(dioProvider.future));
+});
+
+class AdminCounts {
+  const AdminCounts({
+    required this.requests,
+    required this.quarantine,
+    required this.users,
+  });
+  final int requests;
+  final int quarantine;
+  final int users;
+}
+
+/// Fans out to the three list endpoints in parallel and rolls them up
+/// into `{requests, quarantine, users}` for the landing screen badges.
+final adminCountsProvider = FutureProvider<AdminCounts>((ref) async {
+  final requestsApi = await ref.watch(adminRequestsApiProvider.future);
+  final quarantineApi = await ref.watch(adminQuarantineApiProvider.future);
+  final usersApi = await ref.watch(adminUsersApiProvider.future);
+  final results = await Future.wait([
+    requestsApi.list(),
+    quarantineApi.list(),
+    usersApi.list(),
+  ]);
+  return AdminCounts(
+    requests: (results[0] as List).length,
+    quarantine: (results[1] as List).length,
+    users: (results[2] as List).length,
+  );
+});