feat(cache): Add lightweight in-memory cache with TTL and LRU eviction

lib/core/providers/storage_providers.dart

@@ -0,0 +1,35 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+import '../persistence/persistence_providers.dart';
+import '../services/optimized_storage_service.dart';
+import '../services/worker_manager.dart';
+
+/// Provides a shared [FlutterSecureStorage] instance with platform-specific
+/// configuration.
+final secureStorageProvider = Provider<FlutterSecureStorage>((ref) {
+  return const FlutterSecureStorage(
+    aOptions: AndroidOptions(
+      encryptedSharedPreferences: true,
+      sharedPreferencesName: 'conduit_secure_prefs',
+      preferencesKeyPrefix: 'conduit_',
+      // Avoid auto-wipe on transient errors; handled at call sites instead.
+      resetOnError: false,
+    ),
+    iOptions: IOSOptions(
+      accountName: 'conduit_secure_storage',
+      synchronizable: false,
+    ),
+  );
+});
+
+/// Optimized storage service backed by Hive plus secure storage.
+final optimizedStorageServiceProvider = Provider<OptimizedStorageService>((
+  ref,
+) {
+  return OptimizedStorageService(
+    secureStorage: ref.watch(secureStorageProvider),
+    boxes: ref.watch(hiveBoxesProvider),
+    workerManager: ref.watch(workerManagerProvider),
+  );
+});