feat(ops): allow retention days = 0 to wipe table on each scheduled cleanup

Background / 背景

The ops cleanup task currently rejects retention days < 1 in both validate
and normalize, so operators who want minimal-history setups (e.g. high
churn deployments that prefer near-realtime cleanup) cannot express that
intent through the UI. The only options are 1+ days, which keeps at least
24h of history regardless of cron frequency.

ops 清理任务目前在 validate 和 normalize 两处都拒绝小于 1 的保留天数，
让希望尽量不留历史的运维场景（高吞吐部署 + 想用近实时清理）无法通过 UI
表达。最低只能配 1，等于不管 cron 多频繁，至少都会保留 24 小时的历史。

Purpose / 目的

Let admins set retention days to 0, meaning "every scheduled cleanup
run wipes the corresponding table(s) entirely". Combined with a more
frequent cron (e.g. `0 * * * *`) this yields effectively rolling cleanup.

允许管理员把保留天数设为 0，语义为"每次定时清理时把对应表全部清空"。
搭配更频繁的 cron（比如每小时整点）即可获得近似滚动清理的效果。

Changes / 改动内容

Backend

- service/ops_settings.go: validate accepts [0, 365]; normalize only
  refills default 30 when value is < 0 (negative is treated as legacy
  bad data, 0 is honoured)
- service/ops_cleanup_service.go: introduce `opsCleanupPlan(now, days)`
  returning `(cutoff, truncate, ok)`. days==0 returns truncate=true and
  short-circuits to a new `truncateOpsTable` helper that uses
  `TRUNCATE TABLE` (O(1), no WAL, no VACUUM pressure). days>0 keeps
  the existing batched DELETE path unchanged. Empty tables skip
  TRUNCATE to avoid the ACCESS EXCLUSIVE lock entirely
- Extract `isMissingRelationError` helper to dedupe the "table not
  yet created" tolerance shared by both delete and truncate paths
- Add unit tests for `opsCleanupPlan` (three branches) and
  `isMissingRelationError`

后端

- service/ops_settings.go: validate 接受 [0, 365]；normalize 仅在 < 0
  时回填默认 30（负数视为脏数据，0 被尊重）
- service/ops_cleanup_service.go: 抽 `opsCleanupPlan(now, days)` 返回
  `(cutoff, truncate, ok)`。days==0 → truncate=true，走新增
  `truncateOpsTable`（TRUNCATE TABLE，O(1)，无 WAL、无 VACUUM 压力）；
  days>0 仍走原批量 DELETE 路径，行为完全不变。空表跳过 TRUNCATE，
  避免无意义的 ACCESS EXCLUSIVE 锁
- 抽 `isMissingRelationError` helper 复用 delete / truncate 两处的
  "表不存在"宽容判断
- 补 `opsCleanupPlan` 三分支 + `isMissingRelationError` 单元测试

Frontend

- OpsSettingsDialog.vue: validation accepts [0, 365]; input min=0
- i18n (zh/en): hint mentions "0 = wipe all on every cleanup",
  validation message updated to 0-365 range

前端

- OpsSettingsDialog.vue: 校验放宽到 [0, 365]，input min 改 0
- i18n（zh/en）：hint 补"0 = 每次清理时清空所有"，错误提示改 0-365

Trade-offs / 取舍

- TRUNCATE requires ACCESS EXCLUSIVE lock briefly, but ops tables only
  have the cleanup task as a writer, so the lock is invisible to other
  workloads
- Empty-table guard avoids the lock when there is nothing to clean
- Negative values are still treated as legacy bad data and replaced
  with default 30 to preserve compatibility

backend/internal/service/ops_settings.go

@@ -387,13 +387,15 @@ func normalizeOpsAdvancedSettings(cfg *OpsAdvancedSettings) {
 	if cfg.DataRetention.CleanupSchedule == "" {
 		cfg.DataRetention.CleanupSchedule = "0 2 * * *"
 	}
-	if cfg.DataRetention.ErrorLogRetentionDays <= 0 {
+	// 保留天数：0 表示每次定时清理全部（清空所有），> 0 表示按天数保留；
+	// 仅在拿到非法的负数时回填默认值，避免覆盖用户主动设的 0。
+	if cfg.DataRetention.ErrorLogRetentionDays < 0 {
 		cfg.DataRetention.ErrorLogRetentionDays = 30
 	}
-	if cfg.DataRetention.MinuteMetricsRetentionDays <= 0 {
+	if cfg.DataRetention.MinuteMetricsRetentionDays < 0 {
 		cfg.DataRetention.MinuteMetricsRetentionDays = 30
 	}
-	if cfg.DataRetention.HourlyMetricsRetentionDays <= 0 {
+	if cfg.DataRetention.HourlyMetricsRetentionDays < 0 {
 		cfg.DataRetention.HourlyMetricsRetentionDays = 30
 	}
 	// Normalize auto refresh interval (default 30 seconds)
@@ -406,14 +408,15 @@ func validateOpsAdvancedSettings(cfg *OpsAdvancedSettings) error {
 	if cfg == nil {
 		return errors.New("invalid config")
 	}
-	if cfg.DataRetention.ErrorLogRetentionDays < 1 || cfg.DataRetention.ErrorLogRetentionDays > 365 {
-		return errors.New("error_log_retention_days must be between 1 and 365")
+	// 保留天数：0 表示每次清理全部，1-365 表示按天数保留。
+	if cfg.DataRetention.ErrorLogRetentionDays < 0 || cfg.DataRetention.ErrorLogRetentionDays > 365 {
+		return errors.New("error_log_retention_days must be between 0 and 365")
 	}
-	if cfg.DataRetention.MinuteMetricsRetentionDays < 1 || cfg.DataRetention.MinuteMetricsRetentionDays > 365 {
-		return errors.New("minute_metrics_retention_days must be between 1 and 365")
+	if cfg.DataRetention.MinuteMetricsRetentionDays < 0 || cfg.DataRetention.MinuteMetricsRetentionDays > 365 {
+		return errors.New("minute_metrics_retention_days must be between 0 and 365")
 	}
-	if cfg.DataRetention.HourlyMetricsRetentionDays < 1 || cfg.DataRetention.HourlyMetricsRetentionDays > 365 {
-		return errors.New("hourly_metrics_retention_days must be between 1 and 365")
+	if cfg.DataRetention.HourlyMetricsRetentionDays < 0 || cfg.DataRetention.HourlyMetricsRetentionDays > 365 {
+		return errors.New("hourly_metrics_retention_days must be between 0 and 365")
 	}
 	if cfg.AutoRefreshIntervalSec < 15 || cfg.AutoRefreshIntervalSec > 300 {
 		return errors.New("auto_refresh_interval_seconds must be between 15 and 300")