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>

services/cron/tasks_actions.go

@@ -20,6 +20,7 @@ func initActionsTasks() {
 	registerCancelAbandonedJobs()
 	registerScheduleTasks()
 	registerActionsCleanup()
+	registerCleanupActionRuns()
 }
 
 func registerStopZombieTasks() {
@@ -74,3 +75,13 @@ func registerActionsCleanup() {
 		return actions_service.Cleanup(ctx)
 	})
 }
+
+func registerCleanupActionRuns() {
+	RegisterTaskFatal("cleanup_action_runs", &BaseConfig{
+		Enabled:    true,
+		RunAtStart: false,
+		Schedule:   "@midnight",
+	}, func(ctx context.Context, _ *user_model.User, _ *BaseConfig) error {
+		return actions_service.CleanupOldRuns(ctx)
+	})
+}