feat(actions)!: add RUN_RETENTION_DAYS to delete old action runs (#38855)

Gitea keeps completed Actions runs forever. Artifacts and logs expire on
their own schedule, but the run rows never go away, so `action_run` and
its child tables grow without bound.

Adds `RUN_RETENTION_DAYS` to delete completed runs along with their
jobs, tasks and anything the earlier expiries left behind. It defaults
to 400 days, matching how long GitHub keeps run history browsable. A
dedicated `cleanup_action_runs` cron task performs the cleanup, so
admins can schedule it separately from the nightly artifact and log
sweep.

`0` now means "keep forever" for all three retention settings, where
`LOG_RETENTION_DAYS` and `ARTIFACT_RETENTION_DAYS` previously took it
literally and deleted everything at the next sweep.

Docs: https://gitea.com/gitea/docs/pulls/502

----

## ⚠️ BREAKING ⚠️

`RUN_RETENTION_DAYS` defaults to 400, so completed runs older than that
are deleted when the cron task next runs at midnight. Set
`RUN_RETENTION_DAYS = 0` before upgrading to keep all runs.

---------

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

options/locale/locale_en-US.json

@@ -2997,6 +2997,7 @@
   "admin.dashboard.cleanup_hook_task_table": "Clean up hook_task table",
   "admin.dashboard.cleanup_packages": "Clean up expired packages",
   "admin.dashboard.cleanup_actions": "Clean up expired actions' resources",
+  "admin.dashboard.cleanup_action_runs": "Delete action runs older than retention period",
   "admin.dashboard.server_uptime": "Server Uptime",
   "admin.dashboard.current_goroutine": "Current Goroutines",
   "admin.dashboard.current_memory_usage": "Current Memory Usage",