Refuse workflows that move cards in a loop

Every enabled workflow is an edge "source stack -> target stack";
LoopDetector finds circles in that graph, a workflow whose source is its
own target being the shortest. Without a check, two workflows A -> B and
B -> A move the same cards back and forth on every run, even within a
single run, with a notice for each move. The trigger date is ignored on
purpose: by start date one way and by due date the other, a card with
both dates passed still circles. Chains stay allowed.

The controller rejects saving an enabled workflow that would close a loop
and names the workflows involved. Workflows stored before this check are
skipped by the runner (logged, not disabled) and flagged in the settings
list via a new `loop` field from index().

lib/Service/WorkflowRunner.php

@@ -67,11 +67,25 @@ class WorkflowRunner {
 			return;
 		}
 
+		// The controller refuses to store a loop, but workflows saved before
+		// that check — including ones whose source is their own target — can
+		// still form one. They would move the same cards on every run, with a
+		// notice each time, so they are left out until the user fixes them;
+		// the settings page marks them.
+		$looping = LoopDetector::findLoopingWorkflows(LoopDetector::edgesOf($workflows));
+
 		$this->pendingNotices = [];
 		$byUser = [];
 		$owners = [];
 		foreach ($workflows as $workflow) {
 			$this->followBoardOwner($workflow, $owners);
+			if (in_array((int)$workflow->getId(), $looping, true)) {
+				$this->logger->warning('Skipping workflow {id}: it moves cards in a loop together with other workflows', [
+					'app' => 'workflow_deck_automation',
+					'id' => $workflow->getId(),
+				]);
+				continue;
+			}
 			$byUser[$workflow->getUserId()][] = $workflow;
 		}