admin: explain detection cooldowns and skipped scans (#12185)

* plugin: describe detection cooldowns and report skipped scans

* admin: expose cooldown eligibility and preserve skipped detection outcomes

* admin: show cooldown eligibility alongside maintenance lock waits

* plugin: retain cooldown activity for older admin servers

* admin: keep manual detection skips out of scheduler run state

* plugin: refresh descriptors on reconnect without downgrading saved schemas

* plugin: allow valid schema refreshes to repair corrupt descriptors

* admin: store eligible_after activity detail as RFC3339

* worker: report admin_script interval skips as skipped detection

* worker: report admin_script interval skips after the last completed job

* admin: show the selected job's cooldown while scheduling is disabled

* admin: report an unsigned interval skip instead of a completed scan

* worker: populate new DetectionComplete fields in Rust senders

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/plugin/worker/admin_script_handler_test.go

@@ -92,11 +92,14 @@ func TestAdminScriptDetectSkipsByRunInterval(t *testing.T) {
 	if sender.complete == nil || !sender.complete.Success {
 		t.Fatalf("expected successful completion message")
 	}
+	if sender.complete.SkippedReason == "" || sender.complete.EligibleAfter == nil {
+		t.Fatalf("expected skipped completion metadata, got %+v", sender.complete)
+	}
 	if len(sender.events) == 0 {
 		t.Fatalf("expected detector activity events")
 	}
-	if !strings.Contains(sender.events[0].Message, "run interval") {
-		t.Fatalf("unexpected skip message: %q", sender.events[0].Message)
+	if sender.events[0].Stage != "skipped_by_interval" {
+		t.Fatalf("unexpected skip stage: %q", sender.events[0].Stage)
 	}
 }