feat(backup): 新增差异备份（differential）模式 (#88)

文件备份新增差异模式：仅打包自上次全量以来的变更并记录删除，恢复自动按全量+差异链还原。含基线解析、链式恢复、保留链保护与本机文件任务校验；清单/比对/删除/往返/保留保护单测全覆盖。

server/internal/backup/retention/differential_test.go

@@ -0,0 +1,55 @@
+package retention
+
+import (
+	"testing"
+
+	"backupx/server/internal/model"
+)
+
+func retentionRecIDs(records []model.BackupRecord) []uint {
+	ids := make([]uint, 0, len(records))
+	for _, r := range records {
+		ids = append(ids, r.ID)
+	}
+	return ids
+}
+
+// 基线全量仍被「不在删除集合中的差异」依赖 → 必须保留，否则差异无法恢复。
+func TestProtectDifferentialBasesKeepsBaseWithSurvivingDiff(t *testing.T) {
+	all := []model.BackupRecord{
+		{ID: 1, BackupKind: model.BackupKindFull},
+		{ID: 2, BackupKind: model.BackupKindDifferential, BaseRecordID: 1},
+	}
+	candidates := []model.BackupRecord{{ID: 1, BackupKind: model.BackupKindFull}}
+	if got := protectDifferentialBases(all, candidates); len(got) != 0 {
+		t.Fatalf("base with surviving diff must be protected, got %v", retentionRecIDs(got))
+	}
+}
+
+// 基线全量与其全部差异都在删除集合中 → 可一并删除（无残留差异失去基线）。
+func TestProtectDifferentialBasesDeletesBaseWhenDiffAlsoDeleted(t *testing.T) {
+	all := []model.BackupRecord{
+		{ID: 1, BackupKind: model.BackupKindFull},
+		{ID: 2, BackupKind: model.BackupKindDifferential, BaseRecordID: 1},
+	}
+	candidates := []model.BackupRecord{
+		{ID: 1, BackupKind: model.BackupKindFull},
+		{ID: 2, BackupKind: model.BackupKindDifferential, BaseRecordID: 1},
+	}
+	if got := protectDifferentialBases(all, candidates); len(got) != 2 {
+		t.Fatalf("base+diff both expired should both be deleted, got %v", retentionRecIDs(got))
+	}
+}
+
+// 无差异备份时原样透传（不影响既有全量保留逻辑）。
+func TestProtectDifferentialBasesNoDiffsPassThrough(t *testing.T) {
+	all := []model.BackupRecord{
+		{ID: 1, BackupKind: model.BackupKindFull},
+		{ID: 2, BackupKind: model.BackupKindFull},
+	}
+	candidates := []model.BackupRecord{{ID: 1, BackupKind: model.BackupKindFull}}
+	got := protectDifferentialBases(all, candidates)
+	if len(got) != 1 || got[0].ID != 1 {
+		t.Fatalf("no diffs should pass through unchanged, got %v", retentionRecIDs(got))
+	}
+}