fix: address review comments on admin_script seeding

- Replace TOCTOU race (separate Load+Save) with atomic
  SaveJobTypeConfigIfNotExists on ConfigStore and Plugin
- Replace ineffective polling loop with single GetMaster call using
  30s context timeout, since GetMaster respects context cancellation
- Add unit tests for SaveJobTypeConfigIfNotExists (in-memory + on-disk)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/admin/dash/admin_server.go

@@ -287,12 +287,12 @@ func (s *AdminServer) seedAdminScriptFromMaster() {
 		return
 	}
 
-	// Wait for master connection to be available
-	for i := 0; i < 30; i++ {
-		if s.masterClient.GetMaster(context.Background()) != "" {
-			break
-		}
-		time.Sleep(time.Second)
+	// Wait for master connection with a bounded timeout
+	waitCtx, waitCancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer waitCancel()
+	if s.masterClient.GetMaster(waitCtx) == "" {
+		glog.V(1).Infof("Timed out waiting for master connection for admin_script seeding")
+		return
 	}
 
 	var maintenanceScripts string
@@ -318,16 +318,6 @@ func (s *AdminServer) seedAdminScriptFromMaster() {
 		return
 	}
 
-	// Only seed if the admin_script plugin does not already have a saved config
-	existing, err := s.plugin.LoadJobTypeConfig("admin_script")
-	if err != nil {
-		glog.Warningf("Failed to check admin_script plugin config: %v", err)
-		return
-	}
-	if existing != nil {
-		return
-	}
-
 	interval := int64(sleepMinutes)
 	if interval <= 0 {
 		interval = clustermaintenance.DefaultMaintenanceSleepMinutes
@@ -355,11 +345,14 @@ func (s *AdminServer) seedAdminScriptFromMaster() {
 		UpdatedBy: "master_migration",
 	}
 
-	if err := s.plugin.SaveJobTypeConfig(cfg); err != nil {
+	saved, err := s.plugin.SaveJobTypeConfigIfNotExists(cfg)
+	if err != nil {
 		glog.Warningf("Failed to seed admin_script plugin config from master: %v", err)
 		return
 	}
-	glog.V(0).Infof("Seeded admin_script plugin config from master maintenance scripts (interval=%dm)", interval)
+	if saved {
+		glog.V(0).Infof("Seeded admin_script plugin config from master maintenance scripts (interval=%dm)", interval)
+	}
 }
 
 // cleanMaintenanceScript strips lock/unlock commands and normalizes a