Report the latest failed suggestion batch on the deck

The deck reported only a pending or running batch, so a Discover page
opened after a batch failed (or was interrupted by a restart) had no job
to follow and showed "All caught up" or the remaining cards with no
error. GET /api/v1/suggestions/deck now also returns failed_batch: the
newest batch when it failed, read in the same snapshot after orphaned
batches are failed, so at most one of active_batch and failed_batch is
set and a later batch supersedes the failure. The page follows the
failed batch's job like the active one, so its failure shows with the
existing Retry, which asks for a new batch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/App.test.tsx

@@ -107,7 +107,7 @@ describe('App routes', () => {
     mockViewport(390)
     stubFetchByPath({
       ...SIGNED_IN,
-      '/api/v1/suggestions/deck': () => jsonResponse(200, { suggestions: [], active_batch: null }),
+      '/api/v1/suggestions/deck': () => jsonResponse(200, { suggestions: [], active_batch: null, failed_batch: null }),
       '/api/v1/dinners?status=quest_log': () => jsonResponse(200, { dinners: [] }),
     })
     renderAt('/discover')