fix: 按 review 意见重构数据库备份服务（安全性 + 架构 + 健壮性）

1. S3 凭证加密存储：使用 SecretEncryptor (AES-256-GCM) 加密 SecretAccessKey，
   防止备份文件中泄露 S3 凭证，兼容旧的未加密数据
2. 修复 saveRecord 竞态条件：添加 recordsMu 互斥锁保护 records 的 load/save
3. 恢复操作增加服务端验证：handler 层要求重新输入管理员密码，通过 bcrypt
   校验，前端弹出密码输入框
4. pg_dump/psql/S3 操作抽象为接口：定义 DBDumper 和 BackupObjectStore 接口，
   实现放入 repository 层，遵循项目依赖注入架构规范
5. 改为流式处理避免大数据库 OOM：备份时 pg_dump stdout -> gzip -> io.Pipe ->
   S3 upload；恢复时 S3 download -> gzip reader -> psql stdin，不再全量加载
6. loadRecords 区分"无数据"和"数据损坏"场景：JSON 解析失败返回明确错误
7. 添加 18 个核心逻辑单元测试：覆盖加密、并发、流式备份/恢复、错误处理等

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/wire.go

@@ -323,8 +323,14 @@ func ProvideAPIKeyAuthCacheInvalidator(apiKeyService *APIKeyService) APIKeyAuthC
 }
 
 // ProvideBackupService creates and starts BackupService
-func ProvideBackupService(settingRepo SettingRepository, cfg *config.Config) *BackupService {
-	svc := NewBackupService(settingRepo, cfg)
+func ProvideBackupService(
+	settingRepo SettingRepository,
+	cfg *config.Config,
+	encryptor SecretEncryptor,
+	storeFactory BackupObjectStoreFactory,
+	dumper DBDumper,
+) *BackupService {
+	svc := NewBackupService(settingRepo, cfg, encryptor, storeFactory, dumper)
 	svc.Start()
 	return svc
 }