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/repository/backup_pg_dumper.go

@@ -0,0 +1,98 @@
+package repository
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os/exec"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+// PgDumper implements service.DBDumper using pg_dump/psql
+type PgDumper struct {
+	cfg *config.DatabaseConfig
+}
+
+// NewPgDumper creates a new PgDumper
+func NewPgDumper(cfg *config.Config) service.DBDumper {
+	return &PgDumper{cfg: &cfg.Database}
+}
+
+// Dump executes pg_dump and returns a streaming reader of the output
+func (d *PgDumper) Dump(ctx context.Context) (io.ReadCloser, error) {
+	args := []string{
+		"-h", d.cfg.Host,
+		"-p", fmt.Sprintf("%d", d.cfg.Port),
+		"-U", d.cfg.User,
+		"-d", d.cfg.DBName,
+		"--no-owner",
+		"--no-acl",
+		"--clean",
+		"--if-exists",
+	}
+
+	cmd := exec.CommandContext(ctx, "pg_dump", args...)
+	if d.cfg.Password != "" {
+		cmd.Env = append(cmd.Environ(), "PGPASSWORD="+d.cfg.Password)
+	}
+	if d.cfg.SSLMode != "" {
+		cmd.Env = append(cmd.Environ(), "PGSSLMODE="+d.cfg.SSLMode)
+	}
+
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("create stdout pipe: %w", err)
+	}
+
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("start pg_dump: %w", err)
+	}
+
+	// 返回一个 ReadCloser：读 stdout，关闭时等待进程退出
+	return &cmdReadCloser{ReadCloser: stdout, cmd: cmd}, nil
+}
+
+// Restore executes psql to restore from a streaming reader
+func (d *PgDumper) Restore(ctx context.Context, data io.Reader) error {
+	args := []string{
+		"-h", d.cfg.Host,
+		"-p", fmt.Sprintf("%d", d.cfg.Port),
+		"-U", d.cfg.User,
+		"-d", d.cfg.DBName,
+		"--single-transaction",
+	}
+
+	cmd := exec.CommandContext(ctx, "psql", args...)
+	if d.cfg.Password != "" {
+		cmd.Env = append(cmd.Environ(), "PGPASSWORD="+d.cfg.Password)
+	}
+	if d.cfg.SSLMode != "" {
+		cmd.Env = append(cmd.Environ(), "PGSSLMODE="+d.cfg.SSLMode)
+	}
+
+	cmd.Stdin = data
+
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("%v: %s", err, string(output))
+	}
+	return nil
+}
+
+// cmdReadCloser wraps a command stdout pipe and waits for the process on Close
+type cmdReadCloser struct {
+	io.ReadCloser
+	cmd *exec.Cmd
+}
+
+func (c *cmdReadCloser) Close() error {
+	// Close the pipe first
+	_ = c.ReadCloser.Close()
+	// Wait for the process to exit
+	if err := c.cmd.Wait(); err != nil {
+		return fmt.Errorf("pg_dump exited with error: %w", err)
+	}
+	return nil
+}