Expire stuck plugin jobs (#8492)

* Add stale job expiry and expire API

* Add expire job button

* Add test hook and coverage for ExpirePluginJobAPI

* Document scheduler filtering side effect and reuse helper

* Restore job spec proposal test

* Regenerate plugin template output

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Co-authored-by: Copilot <copilot@github.com>

weed/admin/plugin/plugin.go

@@ -80,6 +80,8 @@ type Plugin struct {
 
 	jobsMu sync.RWMutex
 	jobs   map[string]*TrackedJob
+	// serialize stale job cleanup to avoid duplicate expirations
+	staleJobsMu sync.Mutex
 
 	jobDetailsMu sync.Mutex