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/WorkflowRunner.php

@@ -159,6 +159,11 @@ class WorkflowRunner {
 
 		$filterUserIds = $workflow->getFilterUserIdsArray();
 		$filterLabelIds = $workflow->getFilterLabelIdsArray();
+		$this->warnAboutDeadFilters($workflow, $filterUserIds, $filterLabelIds);
+
+		// Resolved at most once per run, and only if a card actually moves
+		// and the workflow wants a mail — this is decoration, not a check.
+		$targets = null;
 
 		foreach ($cards as $card) {
 			if (!self::isOverdue($card->getDaysUntilDue())) {
@@ -171,7 +176,18 @@ class WorkflowRunner {
 				continue;
 			}
 
-			$this->moveAndNotify($user, $workflow, $card);
+			if (!$this->moveCard($workflow, $card)) {
+				continue;
+			}
+
+			if ($workflow->getNotifyEmail()) {
+				$targets ??= $this->deckService->describeTargets(
+					$workflow->getBoardId(),
+					$workflow->getSourceStackId(),
+					$workflow->getTargetStackId(),
+				);
+				$this->mailer->sendCardMovedNotification($user, $workflow, $card, $targets);
+			}
 		}
 
 		$workflow->setLastRun($this->timeFactory->getDateTime());
@@ -226,7 +242,7 @@ class WorkflowRunner {
 		$this->mailer->sendWorkflowDisabledNotification($user, $workflow, $brokenTarget);
 	}
 
-	private function moveAndNotify(IUser $user, Workflow $workflow, Card $card): void {
+	private function moveCard(Workflow $workflow, Card $card): bool {
 		try {
 			$this->deckService->moveCard($card->getId(), $workflow->getTargetStackId());
 		} catch (Throwable $e) {
@@ -235,11 +251,55 @@ class WorkflowRunner {
 				'card' => $card->getId(),
 				'id' => $workflow->getId(),
 			]);
+			return false;
+		}
+
+		return true;
+	}
+
+	/**
+	 * A filter whose every entry has been deleted from the board is not an
+	 * error — the workflow still runs, it just cannot match a single card any
+	 * more, forever, without anything in the UI or the log saying so. That is
+	 * the one failure mode of this app that is completely invisible, so it
+	 * gets a log line on every run.
+	 *
+	 * Explicitly *not* a reason to disable the workflow or mail the user: a
+	 * filter with three labels of which one was deleted still works as
+	 * intended, and even a fully dead filter may be one board edit away from
+	 * being live again. The settings page shows the same condition in red.
+	 *
+	 * @param string[] $filterUserIds
+	 * @param int[] $filterLabelIds
+	 */
+	private function warnAboutDeadFilters(Workflow $workflow, array $filterUserIds, array $filterLabelIds): void {
+		if ($filterUserIds === [] && $filterLabelIds === []) {
 			return;
 		}
 
-		if ($workflow->getNotifyEmail()) {
-			$this->mailer->sendCardMovedNotification($user, $workflow, $card);
+		try {
+			$options = $this->deckService->findFilterOptions($workflow->getBoardId());
+		} catch (DeckUnavailableException $e) {
+			// Same rule as everywhere else: no answer is not a bad answer.
+			$this->logger->debug('Could not check the filters of workflow {id}: ' . $e->getMessage(), [
+				'app' => 'workflow_deck_automation',
+				'id' => $workflow->getId(),
+			]);
+			return;
+		}
+
+		$dead = self::findDeadFilters($filterUserIds, $filterLabelIds, $options['participantUids'], $options['labelIds']);
+
+		if ($dead !== []) {
+			$this->logger->warning(
+				'Workflow {id} can no longer match any card: none of its ' . implode('/', $dead)
+				. ' filter entries exists on board {board} any more',
+				[
+					'app' => 'workflow_deck_automation',
+					'id' => $workflow->getId(),
+					'board' => $workflow->getBoardId(),
+				],
+			);
 		}
 	}
 
@@ -247,6 +307,38 @@ class WorkflowRunner {
 		return $daysUntilDue !== null && $daysUntilDue < 0;
 	}
 
+	/**
+	 * Which filters cannot match anything any more, because not one of their
+	 * entries is still offered by the board. An empty filter is not dead, it
+	 * is "no restriction on this dimension" — the same reading as in
+	 * cardMatchesFilters(), and getting it backwards would report every
+	 * unfiltered workflow as broken.
+	 *
+	 * Static and side-effect free for the same reason as the matching logic:
+	 * testable without a Deck installation.
+	 *
+	 * @param string[] $filterUserIds
+	 * @param int[] $filterLabelIds
+	 * @param string[] $boardUserIds everyone who can hold a card on the board
+	 * @param int[] $boardLabelIds the board's remaining labels
+	 * @return string[] any of 'label', 'user', in that order
+	 */
+	public static function findDeadFilters(
+		array $filterUserIds,
+		array $filterLabelIds,
+		array $boardUserIds,
+		array $boardLabelIds,
+	): array {
+		$dead = [];
+		if ($filterLabelIds !== [] && array_intersect($filterLabelIds, $boardLabelIds) === []) {
+			$dead[] = 'label';
+		}
+		if ($filterUserIds !== [] && array_intersect($filterUserIds, $boardUserIds) === []) {
+			$dead[] = 'user';
+		}
+		return $dead;
+	}
+
 	/**
 	 * Pure filter-matching logic (kept static/side-effect free so it can
 	 * be unit tested without real Deck objects). Both filters are OR