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.

src/PersonalSettings.vue

@@ -159,7 +159,10 @@ const loading = ref(true)
 const loadError = ref('')
 const workflows = ref([])
 const boards = ref([])
+// Per board, and `null` whenever the request failed — see loadStacksFor().
 const stacksByBoard = reactive({})
+const labelsByBoard = reactive({})
+const participantsByBoard = reactive({})
 
 const showForm = ref(false)
 const saving = ref(false)
@@ -181,9 +184,15 @@ const sourceStackOptions = computed(() => stackOptions.value.filter((option) =>
 const targetStackOptions = computed(() => stackOptions.value.filter((option) => option.value !== form.sourceStackId))
 
 /**
- * Workflows whose board or stacks were deleted in Deck, keyed by id. The
- * background job switches these off by itself, but a row that just stopped
- * working needs to say so instead of looking healthy.
+ * Workflows that stopped working, keyed by id. Two different kinds:
+ *
+ * - Board or stacks deleted in Deck. The background job switches these off
+ *   by itself, but a row that just stopped working needs to say so instead
+ *   of looking healthy.
+ * - Every entry of a filter deleted from the board. Nothing switches those
+ *   off — deliberately, since one deleted label out of three is harmless —
+ *   yet a filter that cannot match a single card any more is otherwise
+ *   completely invisible: the workflow keeps running and moves nothing.
  */
 const workflowIssues = computed(() => {
 	const issues = {}
@@ -195,25 +204,49 @@ const workflowIssues = computed(() => {
 		}
 
 		const stacks = stacksByBoard[workflow.boardId]
-		if (!stacks) {
-			continue
+		if (stacks) {
+			const missing = []
+			if (!stacks.some((stack) => stack.id === workflow.sourceStackId)) {
+				missing.push('Quell-Stapel')
+			}
+			if (!stacks.some((stack) => stack.id === workflow.targetStackId)) {
+				missing.push('Ziel-Stapel')
+			}
+			if (missing.length) {
+				issues[workflow.id] = `${missing.join(' und ')} nicht mehr vorhanden`
+				continue
+			}
 		}
 
-		const missing = []
-		if (!stacks.some((stack) => stack.id === workflow.sourceStackId)) {
-			missing.push('Quell-Stapel')
+		const dead = []
+		if (isDeadFilter(workflow.filterLabelIds, labelsByBoard[workflow.boardId], 'id')) {
+			dead.push('Label-Filter')
 		}
-		if (!stacks.some((stack) => stack.id === workflow.targetStackId)) {
-			missing.push('Ziel-Stapel')
+		if (isDeadFilter(workflow.filterUserIds, participantsByBoard[workflow.boardId], 'uid')) {
+			dead.push('Benutzer-Filter')
 		}
-		if (missing.length) {
-			issues[workflow.id] = `${missing.join(' und ')} nicht mehr vorhanden`
+		if (dead.length > 1) {
+			issues[workflow.id] = `${dead.join(' und ')} treffen keine Karte mehr`
+		} else if (dead.length) {
+			issues[workflow.id] = `${dead[0]} trifft keine Karte mehr`
 		}
 	}
 
 	return issues
 })
 
+/**
+ * True only on positive knowledge: the filter has entries, the board's list
+ * was actually loaded, and not one entry is still on it. A failed request
+ * (null) says nothing and must not turn a healthy row red.
+ */
+function isDeadFilter(selected, available, key) {
+	if (!selected.length || !available) {
+		return false
+	}
+	return !selected.some((value) => available.some((entry) => entry[key] === value))
+}
+
 const form = reactive(emptyForm())
 
 function emptyForm() {
@@ -242,11 +275,27 @@ async function loadStacksFor(boardId) {
 	if (!boardId || stacksByBoard[boardId]) {
 		return
 	}
-	try {
-		stacksByBoard[boardId] = await fetchStacks(boardId)
-	} catch (e) {
-		stacksByBoard[boardId] = []
+	// `null`, not `[]` — "could not ask Deck" has to stay distinguishable
+	// from "this board has none", or a single failed request reports every
+	// stack of the board as deleted.
+	stacksByBoard[boardId] = await fetchStacks(boardId).catch(() => null)
+}
+
+/**
+ * Labels and participants of a board — needed only to judge whether a stored
+ * filter can still match anything, so this is fetched just for the boards of
+ * workflows that actually use filters.
+ */
+async function loadFiltersFor(boardId) {
+	if (!boardId || labelsByBoard[boardId]) {
+		return
 	}
+	const [labels, participants] = await Promise.all([
+		fetchLabels(boardId).catch(() => null),
+		fetchParticipants(boardId).catch(() => null),
+	])
+	labelsByBoard[boardId] = labels
+	participantsByBoard[boardId] = participants
 }
 
 async function loadAll() {
@@ -256,7 +305,13 @@ async function loadAll() {
 		const [loadedWorkflows, loadedBoards] = await Promise.all([fetchWorkflows(), fetchBoards()])
 		workflows.value = loadedWorkflows
 		boards.value = loadedBoards
-		await Promise.all(loadedWorkflows.map((workflow) => loadStacksFor(workflow.boardId)))
+		await Promise.all(loadedWorkflows.flatMap((workflow) => {
+			const pending = [loadStacksFor(workflow.boardId)]
+			if (workflow.filterLabelIds.length || workflow.filterUserIds.length) {
+				pending.push(loadFiltersFor(workflow.boardId))
+			}
+			return pending
+		}))
 	} catch (e) {
 		loadError.value = 'Konnte Workflows oder Deck-Boards nicht laden. Ist die Deck-App aktiviert?'
 	} finally {
@@ -281,15 +336,20 @@ async function loadBoardOptions(boardId) {
 
 	// One failing endpoint must not take the other two dropdowns down with it.
 	const [stacks, labels, participants] = await Promise.all([
-		fetchStacks(boardId).catch(() => []),
-		fetchLabels(boardId).catch(() => []),
-		fetchParticipants(boardId).catch(() => []),
+		fetchStacks(boardId).catch(() => null),
+		fetchLabels(boardId).catch(() => null),
+		fetchParticipants(boardId).catch(() => null),
 	])
 
+	// Feed the overview's caches too, so its red markers agree with what the
+	// form just loaded. `null` propagates on purpose: it means "unknown".
 	stacksByBoard[boardId] = stacks
-	stackOptions.value = stacks.map((stack) => ({ value: stack.id, label: stack.title }))
-	labelOptions.value = labels.map((label) => ({ value: label.id, label: label.title }))
-	participantOptions.value = participants.map((participant) => ({ value: participant.uid, label: participant.displayName }))
+	labelsByBoard[boardId] = labels
+	participantsByBoard[boardId] = participants
+
+	stackOptions.value = (stacks ?? []).map((stack) => ({ value: stack.id, label: stack.title }))
+	labelOptions.value = (labels ?? []).map((label) => ({ value: label.id, label: label.title }))
+	participantOptions.value = (participants ?? []).map((participant) => ({ value: participant.uid, label: participant.displayName }))
 }
 
 /**