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().

DEVELOPMENT.md

@@ -57,6 +57,7 @@ composer run test:unit   # PHPUnit
 Die Unit-Tests decken nur reine Logik ab, ohne Deck-Installation:
 - Datumsvergleich für Fälligkeits- und Startdatum (`WorkflowRunner::hasPassed()`)
 - Auslöser und Begründungstext (`TriggerDate`)
+- Schleifenerkennung (`LoopDetector`)
 - Empfängerregel (`NotificationPolicy::shouldNotify()`)
 - Benachrichtigungsweg (`NotificationPolicy::sendsMail()`/`sendsNotification()`)