feat: 文件存储安全加固 + 认证增强 + 媒体URL保护 + provider 重构

## 后端安全加固
- 新增 UserUploadPathResolver: 用户上传文件路径安全解析, 防目录穿越
- LocalReportFileStorage: 文件存储路径安全加固
- local_account_file_cleanup: 账号删除时文件清理逻辑增强
- AuthService: 认证逻辑增强
- file_endpoints / report_endpoints: 文件访问接口安全加固
- ai_chat_endpoints / doctor_endpoints: 接口安全调整
- Program.cs: 服务注册调整

## 前端认证与媒体
- 新增 authenticated_network_image.dart: 带认证的图片加载组件
- auth_provider: 认证状态管理大幅增强(+173)
- api_client: 网络客户端增强(+124)
- chat_provider: 聊天 provider 重构(+76)
- omron_device_provider: 蓝牙设备 provider 增强(+53)
- sse_handler: SSE 处理增强(+35)
- consultation_provider / data_providers / conversation_history_provider: 调整

## 页面调整
- remaining_pages: 健康档案/饮食记录等页面增强(+115)
- home_page / chat_messages_view: 主页微调
- doctor 端多页微调(consultations/dashboard/followups/patient_detail/profile/report_detail/reports)
- report_pages / settings_pages / notification_prefs_page: 微调
- device_scan_page / diet_capture_page / admin_home_page: 微调

## 测试
- 新增 file_path_security_tests: 文件路径安全测试
- 新增 protected_media_url_test: 媒体URL保护测试
- 新增 user_session_identity_test: 用户会话身份测试
- account_deletion_tests / application_service_tests / auth_tests: 更新

health_app/test/protected_media_url_test.dart

@@ -0,0 +1,34 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:health_app/core/api_client.dart' show baseUrl;
+import 'package:health_app/widgets/authenticated_network_image.dart';
+
+void main() {
+  test('legacy user upload url is converted to protected endpoint', () {
+    const fileName = '22222222-2222-2222-2222-222222222222.jpg';
+
+    expect(
+      protectedMediaUrl('/uploads/users/old-user-id/$fileName'),
+      '$baseUrl/api/files/content/$fileName',
+    );
+  });
+
+  test('protected relative endpoint uses configured API base url', () {
+    expect(
+      protectedMediaUrl('/api/reports/report-id/file'),
+      '$baseUrl/api/reports/report-id/file',
+    );
+  });
+
+  test('external urls are left unchanged', () {
+    const url = 'https://example.com/image.jpg';
+    expect(protectedMediaUrl(url), url);
+    expect(mediaRequiresAuthentication(url), isFalse);
+  });
+
+  test('only configured API media receives the login token', () {
+    expect(
+      mediaRequiresAuthentication('$baseUrl/api/files/content/a.jpg'),
+      isTrue,
+    );
+  });
+}