fix: prevent API client provider auto-dispose causing Ref invalidation

API client providers (coreApiClientProvider, tatlockApiClientProvider) now
use keepAlive: true. This fixes "DioException [unknown]: null" errors on
pages like /security/users where ref.read() was used without subscription.

The AuthInterceptor stores a Ref that became invalid when the provider
auto-disposed after a one-time read.

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

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

lib/core/api/api_client.dart

@@ -9,7 +9,10 @@ import 'api_client_native.dart' if (dart.library.html) 'api_client_web.dart'
 part 'api_client.g.dart';
 
 /// Provides the Dio instance for Core API.
-@riverpod
+///
+/// Uses keepAlive to prevent auto-dispose - the AuthInterceptor stores
+/// a Ref that must remain valid for the lifetime of API requests.
+@Riverpod(keepAlive: true)
 Dio coreApiClient(Ref ref) {
   final options = BaseOptions(
     baseUrl: AppConfig.coreApiUrl,
@@ -33,7 +36,10 @@ Dio coreApiClient(Ref ref) {
 }
 
 /// Provides the Dio instance for Tatlock API.
-@riverpod
+///
+/// Uses keepAlive to prevent auto-dispose - the AuthInterceptor stores
+/// a Ref that must remain valid for the lifetime of API requests.
+@Riverpod(keepAlive: true)
 Dio tatlockApiClient(Ref ref) {
   final options = BaseOptions(
     baseUrl: AppConfig.tatlockApiUrl,