feat(payments): edit the rewarded-ads config from the admin catalog page

The "watch for chips" rewarded payout and its per-day / per-hour caps live in the
shared payments.config row and had no admin surface — they could only be changed by
SQL, contrary to D32 (the rewarded rate is meant to be admin-configurable). Add a
"Rewarded ads" form on the /_gm/catalog page: RewardConfig / SetRewardConfig on the
service (non-negative validated), a setRewardConfig store writer (the singleton
config row), the consoleSetReward handler + POST /_gm/catalog/reward route, and the
form pre-filled from the current config. No migration — the columns already exist.

The reward rate is a config value, not a sellable catalog atom (so a "no-ads forever"
style product is still out of scope by D32/D33).

Test: an integration test sets the config, checks the page pre-fill, and refuses a
negative value.

backend/internal/inttest/catalog_editor_test.go

@@ -124,3 +124,30 @@ func TestConsoleCatalogActiveToggle(t *testing.T) {
 			strings.Contains(body, "toggle-active"), strings.Contains(body, "toggle-archived"))
 	}
 }
+
+// TestConsoleRewardConfig checks the rewarded-ads config form on the catalog page: a POST updates the
+// payout and caps, the page pre-fills the current values, and a negative value is refused.
+func TestConsoleRewardConfig(t *testing.T) {
+	srv, _, pay := bannerServer(t)
+	h := srv.Handler()
+	const origin = "http://admin.test"
+	const reward = "http://admin.test/_gm/catalog/reward"
+
+	// Set the payout and caps.
+	if code, body := consoleDo(h, http.MethodPost, reward, "reward_payout=7&reward_daily=40&reward_hourly=9", origin); code != http.StatusOK || !strings.Contains(body, "Saved") {
+		t.Fatalf("set reward = %d, has 'Saved' = %v", code, strings.Contains(body, "Saved"))
+	}
+	if payout, daily, hourly, err := pay.RewardConfig(context.Background()); err != nil || payout != 7 || daily != 40 || hourly != 9 {
+		t.Fatalf("reward config = %d/%d/%d (err %v), want 7/40/9", payout, daily, hourly, err)
+	}
+
+	// The catalog page renders the form pre-filled with the current payout.
+	if _, body := consoleDo(h, http.MethodGet, "http://admin.test/_gm/catalog", "", ""); !strings.Contains(body, "reward_payout") || !strings.Contains(body, `value="7"`) {
+		t.Error("catalog page did not render the reward form with the current payout")
+	}
+
+	// A negative value is refused (the service validates non-negativity).
+	if code, body := consoleDo(h, http.MethodPost, reward, "reward_payout=-1", origin); code != http.StatusOK || !strings.Contains(body, "non-negative") {
+		t.Errorf("negative payout = %d, has 'non-negative' = %v", code, strings.Contains(body, "non-negative"))
+	}
+}