feat(backup): GFS 分层保留策略（祖父-父-子）+ 任务表单配置 (#85)

新增 GFS 分层保留：按天/周/月/年保留代表性备份，各层级并集；任一>0 启用、全0 维持原策略（兼容），锁定记录豁免。后端 retention 算法+任务字段贯通，前端任务表单 GFS 配置。go test、tsc+vite 通过。

server/internal/backup/retention/service.go

@@ -3,6 +3,7 @@ package retention
 import (
 	"context"
 	"fmt"
+	"sort"
 	"strings"
 	"time"
 
@@ -56,7 +57,13 @@ func (s *Service) Cleanup(ctx context.Context, task *model.BackupTask, provider
 	if err != nil {
 		return nil, fmt.Errorf("list successful records: %w", err)
 	}
-	candidates := selectRecordsToDelete(records, task.RetentionDays, task.MaxBackups, s.now())
+	var candidates []model.BackupRecord
+	if gfsEnabled(task) {
+		// GFS 策略：按天/周/月/年分层保留代表性备份，取代简单的天数/数量策略。
+		candidates = selectGFSToDelete(records, task.KeepDaily, task.KeepWeekly, task.KeepMonthly, task.KeepYearly)
+	} else {
+		candidates = selectRecordsToDelete(records, task.RetentionDays, task.MaxBackups, s.now())
+	}
 	result := &CleanupResult{}
 	for _, record := range candidates {
 		if strings.TrimSpace(record.StoragePath) != "" {
@@ -133,3 +140,72 @@ func hasLocked(records []model.BackupRecord) bool {
 	}
 	return false
 }
+
+// gfsEnabled 判定任务是否启用 GFS 分层保留（任一层级 > 0）。
+func gfsEnabled(task *model.BackupTask) bool {
+	return task.KeepDaily > 0 || task.KeepWeekly > 0 || task.KeepMonthly > 0 || task.KeepYearly > 0
+}
+
+func recordTime(r *model.BackupRecord) time.Time {
+	if r.CompletedAt != nil {
+		return *r.CompletedAt
+	}
+	return r.StartedAt
+}
+
+func isoWeekKey(t time.Time) string {
+	y, w := t.ISOWeek()
+	return fmt.Sprintf("%d-W%02d", y, w)
+}
+
+// selectGFSToDelete 按 GFS（祖父-父-子）策略选出应删除的记录。
+//
+// 规则：对每个层级（天/周/月/年），在按时间降序排列后，保留最近 keep 个不同周期中
+// 每个周期最新的一份备份；各层级保留集合取并集即「保留集」，其余删除。
+// 锁定（法律保留）的记录始终排除在删除候选之外。
+func selectGFSToDelete(records []model.BackupRecord, daily, weekly, monthly, yearly int) []model.BackupRecord {
+	active := make([]model.BackupRecord, 0, len(records))
+	for i := range records {
+		if !records[i].Locked {
+			active = append(active, records[i])
+		}
+	}
+	sort.SliceStable(active, func(i, j int) bool {
+		return recordTime(&active[i]).After(recordTime(&active[j]))
+	})
+
+	keep := make(map[uint]bool, len(active))
+	keepTier := func(count int, key func(time.Time) string) {
+		if count <= 0 {
+			return
+		}
+		periods := 0
+		lastPeriod := ""
+		havePrev := false
+		for i := range active {
+			p := key(recordTime(&active[i]))
+			if havePrev && p == lastPeriod {
+				continue // 同周期已保留代表（最新一份）
+			}
+			if periods >= count {
+				break // 该层级已保留足够多的周期
+			}
+			keep[active[i].ID] = true
+			lastPeriod = p
+			havePrev = true
+			periods++
+		}
+	}
+	keepTier(daily, func(t time.Time) string { return t.Format("2006-01-02") })
+	keepTier(weekly, isoWeekKey)
+	keepTier(monthly, func(t time.Time) string { return t.Format("2006-01") })
+	keepTier(yearly, func(t time.Time) string { return t.Format("2006") })
+
+	del := make([]model.BackupRecord, 0)
+	for i := range active {
+		if !keep[active[i].ID] {
+			del = append(del, active[i])
+		}
+	}
+	return del
+}

server/internal/backup/retention/service_test.go

@@ -93,6 +93,81 @@ func TestSelectRecordsToDelete(t *testing.T) {
 	}
 }
 
+func gfsRecord(id uint, ts time.Time, locked bool) model.BackupRecord {
+	completed := ts
+	return model.BackupRecord{ID: id, StartedAt: ts, CompletedAt: &completed, Locked: locked}
+}
+
+func gfsDay(y, m, d, h int) time.Time {
+	return time.Date(y, time.Month(m), d, h, 0, 0, 0, time.UTC)
+}
+
+func deletedIDSet(records []model.BackupRecord) map[uint]bool {
+	out := make(map[uint]bool, len(records))
+	for i := range records {
+		out[records[i].ID] = true
+	}
+	return out
+}
+
+func assertDeleted(t *testing.T, del []model.BackupRecord, want ...uint) {
+	t.Helper()
+	got := deletedIDSet(del)
+	if len(got) != len(want) {
+		t.Fatalf("deleted set size = %d %v, want %d %v", len(got), got, len(want), want)
+	}
+	for _, id := range want {
+		if !got[id] {
+			t.Fatalf("expected id %d to be deleted; got %v", id, got)
+		}
+	}
+}
+
+// TestSelectGFSToDelete_DailyTier 验证按天分层：每天仅保留最新一份，且只保留最近 N 天。
+func TestSelectGFSToDelete_DailyTier(t *testing.T) {
+	records := []model.BackupRecord{
+		gfsRecord(5, gfsDay(2026, 3, 7, 12), false), // 今天，最新 → 保留
+		gfsRecord(4, gfsDay(2026, 3, 7, 6), false),  // 今天，较早 → 删除（非当天代表）
+		gfsRecord(3, gfsDay(2026, 3, 6, 12), false), // 昨天 → 保留
+		gfsRecord(2, gfsDay(2026, 3, 5, 12), false), // 前天 → 超出 daily=2 → 删除
+		gfsRecord(1, gfsDay(2026, 3, 4, 12), false), // 更早 → 删除
+	}
+	del := selectGFSToDelete(records, 2, 0, 0, 0)
+	assertDeleted(t, del, 4, 2, 1)
+}
+
+// TestSelectGFSToDelete_TierUnion 验证多层级取并集：月度层级保留日度层级会删除的旧备份。
+func TestSelectGFSToDelete_TierUnion(t *testing.T) {
+	records := []model.BackupRecord{
+		gfsRecord(3, gfsDay(2026, 3, 7, 12), false),  // 3 月（最新）
+		gfsRecord(2, gfsDay(2026, 2, 15, 12), false), // 2 月
+		gfsRecord(1, gfsDay(2026, 1, 15, 12), false), // 1 月
+	}
+	// daily=1 只留 ID3；monthly=2 留最近两个月（3 月=ID3、2 月=ID2）。并集={3,2}，删除 ID1。
+	del := selectGFSToDelete(records, 1, 0, 2, 0)
+	assertDeleted(t, del, 1)
+}
+
+// TestSelectGFSToDelete_SkipsLocked 验证锁定记录即使超出所有层级也永不删除。
+func TestSelectGFSToDelete_SkipsLocked(t *testing.T) {
+	records := []model.BackupRecord{
+		gfsRecord(3, gfsDay(2026, 3, 7, 12), false),
+		gfsRecord(2, gfsDay(2026, 3, 6, 12), false),
+		gfsRecord(1, gfsDay(2020, 1, 1, 12), true), // 远超 daily=1 但已锁定 → 不删
+	}
+	del := selectGFSToDelete(records, 1, 0, 0, 0)
+	assertDeleted(t, del, 2) // 仅 ID2 被删；ID1 锁定豁免，ID3 为当日代表
+}
+
+func TestGFSEnabled(t *testing.T) {
+	if gfsEnabled(&model.BackupTask{}) {
+		t.Fatal("empty GFS config should be disabled")
+	}
+	if !gfsEnabled(&model.BackupTask{KeepWeekly: 4}) {
+		t.Fatal("KeepWeekly>0 should enable GFS")
+	}
+}
+
 // TestSelectRecordsToDelete_SkipsLocked 验证保留锁定（法律保留）的记录永不被选中删除，
 // 即使它既超过保留期、又超过 maxBackups 名额。
 func TestSelectRecordsToDelete_SkipsLocked(t *testing.T) {