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>

internal/server/suggestions_test.go

@@ -174,6 +174,9 @@ func TestSuggestionDeckActiveBatch(t *testing.T) {
 	if active, ok := body["active_batch"]; !ok || active != nil {
 		t.Fatalf("deck = %v, want active_batch null", body)
 	}
+	if failed, ok := body["failed_batch"]; !ok || failed != nil {
+		t.Fatalf("deck = %v, want failed_batch null", body)
+	}
 
 	rec, started := callDinners(t, h, http.MethodPost, suggestionsPath+"/batches", "")
 	if rec.Code != http.StatusAccepted {