Surface dead filters, and name board and stacks in the moved-card mail

A filter whose entries have all been deleted from the board was the one
failure mode of this app that was completely invisible: the workflow kept
running, kept updating last_run and matched nothing, forever. It happens
with deleted labels and just as easily when a filtered user loses board
access, because Deck's BoardService::deleteAcl() calls
assignedUsersMapper->deleteByParticipantOnBoard() and wipes that user's
card assignments.

Two low-stakes signals, no new column and no migration:

- WorkflowRunner::warnAboutDeadFilters() logs a warning on every run.
- workflowIssues in PersonalSettings.vue marks the row red.

Deliberately no mail and no `enabled = false`. One deleted label out of
three is harmless, and even a fully dead filter can be one board edit away
from being live again.

Both obey the rule the disable path already follows: only positive
knowledge. DeckIntegrationService::findFilterOptions() therefore throws
instead of degrading to [], unlike listLabels()/listParticipants(), where
an empty list only means an empty dropdown.

That also fixes an existing false positive of the same family:
loadStacksFor() stored [] when the request failed, so a single failed
fetch reported "Quell-Stapel und Ziel-Stapel nicht mehr vorhanden" for an
untouched board. Failed requests now store null and are read as "unknown".

The comparison itself lives in WorkflowRunner::findDeadFilters(), static
and side-effect free like cardMatchesFilters(), with unit tests -- notably
that an empty filter is never dead, which would otherwise flag every
unfiltered workflow.

Separately, the moved-card mail now names the board and both stacks.
DeckIntegrationService::describeTargets() resolves the titles at most once
per workflow run, and only when a card actually moved and the workflow
wants a mail; unreadable titles degrade to #<id> rather than costing the
user their notification.

lib/Service/NotificationMailer.php

@@ -30,7 +30,11 @@ class NotificationMailer {
 	) {
 	}
 
-	public function sendCardMovedNotification(IUser $user, Workflow $workflow, Card $card): void {
+	/**
+	 * @param array{board: string, sourceStack: string, targetStack: string} $targets
+	 *        titles resolved by DeckIntegrationService::describeTargets()
+	 */
+	public function sendCardMovedNotification(IUser $user, Workflow $workflow, Card $card, array $targets): void {
 		$email = $user->getEMailAddress();
 		if ($email === null || $email === '') {
 			$this->logger->info('Skipping notification for workflow ' . $workflow->getId() . ': user ' . $user->getUID() . ' has no email address', [
@@ -55,6 +59,13 @@ class NotificationMailer {
 				'The card "%1$s" was moved because it is overdue (workflow "%2$s").',
 				[$card->getTitle(), $workflow->getTitle()],
 			));
+			// Single-argument addBodyText() runs htmlspecialchars() itself, so
+			// these board- and stack-titles (user input) are safe as-is —
+			// unlike the description below, which passes both parts.
+			$template->addBodyText($this->l10n->t(
+				'Board "%1$s": moved from "%2$s" to "%3$s".',
+				[$targets['board'], $targets['sourceStack'], $targets['targetStack']],
+			));
 			$this->addCardDescription($template, $card);
 			$template->addBodyButton($this->l10n->t('Open card'), $cardLink);
 			$template->addFooter();