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.

tests/Unit/Service/WorkflowRunnerFilterTest.php

@@ -43,4 +43,28 @@ class WorkflowRunnerFilterTest extends TestCase {
 		// both match -> true
 		$this->assertTrue(WorkflowRunner::cardMatchesFilters(['alice'], [1], ['alice'], [1]));
 	}
+
+	public function testEmptyFiltersAreNeverDead(): void {
+		// "no restriction" must not be reported as a broken filter, however
+		// little the board has left to offer.
+		$this->assertSame([], WorkflowRunner::findDeadFilters([], [], [], []));
+		$this->assertSame([], WorkflowRunner::findDeadFilters([], [], ['alice'], [1]));
+	}
+
+	public function testFilterSurvivesAsLongAsOneEntryRemains(): void {
+		$this->assertSame([], WorkflowRunner::findDeadFilters(['alice', 'bob'], [1, 2], ['bob'], [2]));
+	}
+
+	public function testFilterIsDeadWhenEveryEntryIsGone(): void {
+		$this->assertSame(['label'], WorkflowRunner::findDeadFilters([], [1, 2], [], [3]));
+		$this->assertSame(['user'], WorkflowRunner::findDeadFilters(['alice'], [], ['bob'], []));
+		$this->assertSame(
+			['label', 'user'],
+			WorkflowRunner::findDeadFilters(['alice'], [1], ['bob'], [2]),
+		);
+	}
+
+	public function testAnEmptyBoardKillsOnlyTheFiltersThatAreSet(): void {
+		$this->assertSame(['label'], WorkflowRunner::findDeadFilters([], [1], [], []));
+	}
 }