Handle workflows whose board or stack was deleted

Deleting a board or stack left the workflow row pointing at nothing: Deck
keeps the orphaned cards readable until its DeleteCron purges them, so the
run kept failing on the move every few minutes, silently and forever.

- The controller now rejects boards and stacks that do not exist (or are not
  the user's) on create and update, so no new broken row can be stored.
- The settings list marks affected workflows in red instead of showing a row
  that looks healthy.
- The background job disables such a workflow and mails its owner once. The
  mail ignores the notifyEmail flag: that one is about moved cards, this is a
  notice that the automation stopped. It stays a one-off because a disabled
  workflow is no longer picked up.

The check deliberately goes through StackService::findAll() rather than
BoardService::getUserBoards(): Deck injects the current user into BoardService
as a string frozen at construction, so in the job -- one process, many users,
a cached BoardService -- it would answer for the wrong user or for none, and
every workflow on the instance would have been disabled. findStackIds()
returns null only for a genuinely missing or forbidden board and throws for
anything else, so a Deck outage skips the workflow instead of killing it.

lib/Service/DeckIntegrationService.php

@@ -10,6 +10,7 @@ use OCA\Deck\Service\BoardService;
 use OCA\Deck\Service\CardService;
 use OCA\Deck\Service\StackService;
 use OCP\App\IAppManager;
+use OCP\AppFramework\Db\DoesNotExistException;
 use OCP\IGroupManager;
 use OCP\IUser;
 use OCP\IUserManager;
@@ -57,6 +58,13 @@ class DeckIntegrationService {
 
 	/**
 	 * @return array<int, array{id: int, title: string}>
+	 *
+	 * Only ever call this from a *request*: Deck injects the current user id
+	 * into BoardService as a plain string frozen at construction time, so in
+	 * the background job (one process, many users, a container-cached
+	 * BoardService) this would answer for the wrong user — or for none at
+	 * all. The job uses findStackIds() instead, which goes through Deck's
+	 * PermissionService and therefore reads the live session.
 	 */
 	public function listBoardsForCurrentUser(): array {
 		$boards = $this->call(function () {
@@ -110,11 +118,61 @@ class DeckIntegrationService {
 		}, []);
 
 		return array_map(
-			static fn ($stack) => ['id' => $stack->getId(), 'title' => $stack->getTitle()],
+			static fn ($stack) => ['id' => (int)$stack->getId(), 'title' => $stack->getTitle()],
 			$stacks,
 		);
 	}
 
+	/**
+	 * Ids of the board's stacks, for deciding whether a stored workflow still
+	 * points at anything real.
+	 *
+	 * Returns `null` when the board itself is gone or not readable for the
+	 * current user — `StackService::findAll()` runs a Deck permission check
+	 * first, and that one reads the *live* session, which is what makes this
+	 * usable from the impersonating background job. Any other failure throws,
+	 * because "Deck is broken right now" must never be mistaken for "the user
+	 * deleted this board".
+	 *
+	 * @return int[]|null
+	 * @throws DeckUnavailableException
+	 */
+	public function findStackIds(int $boardId): ?array {
+		try {
+			$stacks = $this->resolve(StackService::class)->findAll($boardId);
+		} catch (DeckUnavailableException $e) {
+			throw $e;
+		} catch (Throwable $e) {
+			if ($this->isMissingOrForbidden($e)) {
+				return null;
+			}
+			$this->logger->error('Could not list stacks of board ' . $boardId . ': ' . $e->getMessage(), [
+				'app' => 'workflow_deck_automation',
+				'exception' => $e,
+			]);
+			throw new DeckUnavailableException('Deck call failed: ' . $e->getMessage(), 0, $e);
+		}
+
+		return array_map(static fn ($stack) => (int)$stack->getId(), $stacks);
+	}
+
+	/**
+	 * Deck's own exception classes are referenced by name: `is_a()` with a
+	 * string simply returns false when the class does not exist, so a missing
+	 * or renamed Deck degrades to "unknown error" instead of fataling.
+	 */
+	private function isMissingOrForbidden(Throwable $e): bool {
+		if ($e instanceof DoesNotExistException) {
+			return true;
+		}
+		foreach (['OCA\Deck\NoPermissionException', 'OCA\Deck\NotFoundException'] as $class) {
+			if (is_a($e, $class)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * @return array<int, array{id: int, title: string, color: string|null}>
 	 */