Surface replenish failures, follow the active batch, scope deck keys, keep three deck depths

Fixes the Feature 13 review findings on the Discover deck (Maestro task
4fc551f0-3c11-4c15-b638-f2c08a28eb29):

- F13-01: a batch failure (start, poll or job) is shown with its Retry
  whatever the deck has, so a replenish that fails while cards remain is
  no longer silent until the deck runs out.
- F13-02: GET /api/v1/suggestions/deck reports the batch pending or
  running (active_batch, read in the deck's snapshot), and a decision
  that calls for a replenish returns the job of the batch it started or
  joined. The page follows a newly reported active batch, so a reopened
  page or a second session shows skeleton cards while the batch runs
  instead of "All caught up".
- F13-03: ← / → leave the deck alone while it is inert behind a modal
  (the Quest log sheet or its confirmation).
- F13-04: a one- or two-card deck keeps all three depths: skeleton cards
  stand in for the missing ones while a batch finds more Side quests,
  blank stack-edge cards while none is coming.

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

internal/server/suggestions_test.go

@@ -148,8 +148,44 @@ func TestSuggestionDecisions(t *testing.T) {
 	if rec.Code != http.StatusOK {
 		t.Fatalf("decline = %d: %s", rec.Code, rec.Body)
 	}
-	if id, _ := body["replenish_job_id"].(string); id == "" {
-		t.Errorf("decline leaving 4 = %v, want a replenish_job_id", body)
+	replenish, _ := body["replenish_job_id"].(string)
+	if replenish == "" {
+		t.Fatalf("decline leaving 4 = %v, want a replenish_job_id", body)
+	}
+
+	// A fourth decision joins the batch already pending and reports its job.
+	rec, body = callDinners(t, h, http.MethodPost, suggestionsPath+"/"+ids[3]+"/accept", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("accept = %d: %s", rec.Code, rec.Body)
+	}
+	if body["replenish_job_id"] != replenish {
+		t.Errorf("accept leaving 3 = %v, want replenish_job_id %s", body, replenish)
+	}
+}
+
+func TestSuggestionDeckActiveBatch(t *testing.T) {
+	t.Parallel()
+	h, _, _ := newSuggestionsRouter(t, "Pho")
+
+	rec, body := callDinners(t, h, http.MethodGet, suggestionsPath+"/deck", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("GET deck = %d: %s", rec.Code, rec.Body)
+	}
+	if active, ok := body["active_batch"]; !ok || active != nil {
+		t.Fatalf("deck = %v, want active_batch null", body)
+	}
+
+	rec, started := callDinners(t, h, http.MethodPost, suggestionsPath+"/batches", "")
+	if rec.Code != http.StatusAccepted {
+		t.Fatalf("POST batches = %d: %s", rec.Code, rec.Body)
+	}
+	rec, body = callDinners(t, h, http.MethodGet, suggestionsPath+"/deck", "")
+	if rec.Code != http.StatusOK {
+		t.Fatalf("GET deck = %d: %s", rec.Code, rec.Body)
+	}
+	active, _ := body["active_batch"].(map[string]any)
+	if active == nil || active["batch_id"] != started["batch_id"] || active["job_id"] != started["job_id"] || active["status"] != "pending" {
+		t.Errorf("deck active_batch = %v, want the started batch %v", body["active_batch"], started)
 	}
 }