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

frontend/src/i18n/locales/zh.ts

@@ -4810,7 +4810,7 @@ export default {
         errorLogRetentionDays: '错误日志保留天数',
         minuteMetricsRetentionDays: '分钟指标保留天数',
         hourlyMetricsRetentionDays: '小时指标保留天数',
-        retentionDaysHint: '建议保留7-90天，过长会占用存储空间',
+        retentionDaysHint: '建议保留 7-90 天，过长会占用存储空间；填 0 表示每次定时清理时清空所有历史',
         aggregation: '预聚合任务',
         enableAggregation: '启用预聚合任务',
         aggregationHint: '预聚合可提升长时间窗口查询性能',
@@ -4841,7 +4841,7 @@ export default {
         autoRefreshCountdown: '自动刷新：{seconds}s',
         validation: {
           title: '请先修正以下问题',
-          retentionDaysRange: '保留天数必须在1-365天之间',
+          retentionDaysRange: '保留天数必须在 0-365 天之间（0 = 每次清理时清空所有）',
           slaMinPercentRange: 'SLA最低百分比必须在0-100之间',
           ttftP99MaxRange: 'TTFT P99最大值必须大于等于0',
           requestErrorRateMaxRange: '请求错误率最大值必须在0-100之间',