feat(backup): 备份保留锁定 / 法律保留（豁免清理删除 + 记录页锁定） (#84)

新增保留锁定：锁定的备份豁免保留期清理与手动删除（迁移基线/合规快照/取证）。model+Locked、retention 剔除锁定记录、DeleteRecord 拒绝删除、PUT /backup/records/:id/lock、记录页锁定/解锁操作与标识。go test、tsc+vite、运行时路由验证均通过。

server/internal/backup/retention/service.go

@@ -91,6 +91,17 @@ func (s *Service) Cleanup(ctx context.Context, task *model.BackupTask, provider
 }
 
 func selectRecordsToDelete(records []model.BackupRecord, retentionDays int, maxBackups int, now time.Time) []model.BackupRecord {
+	// 保留锁定（法律保留）的记录永不参与清理：先从候选集中剔除，
+	// 锁定备份既不被删除，也不占用 maxBackups 轮转名额。
+	if hasLocked(records) {
+		unlocked := make([]model.BackupRecord, 0, len(records))
+		for _, r := range records {
+			if !r.Locked {
+				unlocked = append(unlocked, r)
+			}
+		}
+		records = unlocked
+	}
 	selected := make(map[uint]model.BackupRecord)
 	if maxBackups > 0 && len(records) > maxBackups {
 		for _, record := range records[maxBackups:] {
@@ -113,3 +124,12 @@ func selectRecordsToDelete(records []model.BackupRecord, retentionDays int, maxB
 	}
 	return result
 }
+
+func hasLocked(records []model.BackupRecord) bool {
+	for i := range records {
+		if records[i].Locked {
+			return true
+		}
+	}
+	return false
+}