feat: add local-first private AI digest workflow

Migrate app code into canonical feature slices, add phone-only AI digest scheduling and review, wire local notification/background task support, and cover the flow with tests.

lib/features/ai_digest/application/llm_digest_text_client.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:relationship_saver/core/llm/llm_service.dart';
+
+abstract interface class LlmDigestTextClient {
+  Future<String> complete({
+    required String systemPrompt,
+    required String userPrompt,
+  });
+}
+
+class ConfiguredLlmDigestTextClient implements LlmDigestTextClient {
+  const ConfiguredLlmDigestTextClient(this._service);
+
+  final LlmService _service;
+
+  @override
+  Future<String> complete({
+    required String systemPrompt,
+    required String userPrompt,
+  }) {
+    return _service.completeText(
+      systemPrompt: systemPrompt,
+      userPrompt: userPrompt,
+    );
+  }
+}
+
+final Provider<LlmDigestTextClient> llmDigestTextClientProvider =
+    Provider<LlmDigestTextClient>((Ref ref) {
+      return ConfiguredLlmDigestTextClient(ref.watch(llmServiceProvider));
+    });