chore: upgrade to riverpod 3.x and freezed 3.x

- Upgrade flutter_riverpod to 3.1.0, riverpod_annotation to 4.0.0
- Upgrade freezed to 3.2.3, freezed_annotation to 3.1.0
- Migrate freezed classes to use sealed keyword (freezed 3.x)
- Update provider naming (*NotifierProvider → *Provider)
- Add legacy.dart import for StateNotifierProvider compatibility
- Fix valueOrNull → value for AsyncValue
- Remove unused imports and fields
- Add sync from Authentik button to users/groups pages
- Suppress invalid_annotation_target warning in analysis_options

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/features/security/users/users_list_page.dart

@@ -70,6 +70,7 @@ class UsersListPage extends ConsumerStatefulWidget {
 class _UsersListPageState extends ConsumerState<UsersListPage> {
   late final StateNotifierProvider<DataGridController<UserData>,
       DataGridState<UserData>> _gridProvider;
+  bool _isSyncing = false;
 
   @override
   void initState() {
@@ -88,6 +89,38 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
     );
   }
 
+  Future<void> _syncFromAuthentik() async {
+    if (_isSyncing) return;
+    setState(() => _isSyncing = true);
+
+    try {
+      final dio = ref.read(coreApiClientProvider);
+      await dio.post<void>('/auth/users/sync-from-authentik');
+
+      if (!mounted) return;
+      ref.read(_gridProvider.notifier).refresh();
+
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: const Text('Users synced from Authentik'),
+          behavior: SnackBarBehavior.floating,
+          duration: const Duration(seconds: 2),
+        ),
+      );
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text('Sync failed: $e'),
+          behavior: SnackBarBehavior.floating,
+          duration: const Duration(seconds: 4),
+        ),
+      );
+    } finally {
+      if (mounted) setState(() => _isSyncing = false);
+    }
+  }
+
   DataGridConfig<UserData> _buildConfig() {
     return DataGridConfig<UserData>(
       columns: [
@@ -131,6 +164,18 @@ class _UsersListPageState extends ConsumerState<UsersListPage> {
           tooltip: 'Refresh',
           onPressed: () => ref.read(_gridProvider.notifier).refresh(),
         ),
+        const SizedBox(width: 8),
+        FilledButton.icon(
+          onPressed: _isSyncing ? null : _syncFromAuthentik,
+          icon: _isSyncing
+              ? const SizedBox(
+                  width: 16,
+                  height: 16,
+                  child: CircularProgressIndicator(strokeWidth: 2),
+                )
+              : const Icon(Icons.sync),
+          label: const Text('Sync from Authentik'),
+        ),
       ],
     );
   }