修复(#3): 抽奖 GrantLedger.Payload 空字符串违反 MySQL JSON 校验

Closes HIF-3 (Stage 1 P0 from QA smoke - third and final of the same class)

F6 (P0): GrantLedger.Payload=\"\" → MySQL error 3140 (Invalid JSON text: The document is empty)
- 修法：方式 A 对称守卫 Payload=\"{}\" (与 PR E UnmetReasons=\"[]\", PR C PrizeSnapshot.Config=\"{}\" 三处守卫模式统一)
- QA 已 sweep 全部 6 个 lottery JSON 列，这是最后一处漏守
- handler 成功 dispatch 后会 UpdateColumn 覆盖真实 payload；Reserve 阶段用 {} 兜底

回归护栏：TestReserve_EmptyPayloadDefaultsToEmptyJSONObject 用 payloadNotEmptyString per-arg matcher
CI 全绿；2 files, +83/-0

架构师复盘：三次同一 pattern 漏检查（F2 audit ctx keys / F4 UnmetReasons / F6 Payload）。Stage 2 起硬性规则：grep -RIn 'type:json' internal/model/ 全量清单逐列 sweep + 每列至少一条 per-arg matcher 单测。感谢 QA 三次挖坑。

internal/model/lottery/grant_ledger.go

@@ -51,6 +51,13 @@ func (s *ledgerService) Reserve(ctx context.Context, tx *gorm.DB, entry GrantLed
 	if entry.ExternalRef == "" {
 		return nil, false, errors.New("Reserve requires a non-empty ExternalRef")
 	}
+	// Payload 是 JSON 列，MySQL 拒绝空字符串（error 3140）——
+	// handler 在成功 dispatch 后会 UpdateColumn 覆盖真实 payload；Reserve 阶
+	// 段的空 payload 用 "{}" 兜底，与 PrizeSnapshot.Config、
+	// EligibilitySnapshot.UnmetReasons 的守卫对称。
+	if entry.Payload == "" {
+		entry.Payload = "{}"
+	}
 
 	insertRes := tx.WithContext(ctx).
 		Clauses(clause.OnConflict{DoNothing: true}).

internal/model/lottery/grant_ledger_test.go

@@ -2,6 +2,7 @@ package lottery
 
 import (
 	"context"
+	"database/sql/driver"
 	"testing"
 
 	"github.com/DATA-DOG/go-sqlmock"
@@ -110,3 +111,78 @@ func TestLedgerService_Reserve_RequiresExternalRef(t *testing.T) {
 		t.Fatalf("expected error on empty ExternalRef")
 	}
 }
+
+// TestReserve_EmptyPayloadDefaultsToEmptyJSONObject is the F6 regression guard.
+//
+// Before PR F, Reserve created lottery_grant_ledger rows with the caller's
+// empty entry.Payload written verbatim ("") into the `payload` JSON column —
+// MySQL error 3140 rejects empty strings on JSON columns, so every real
+// draw's ledger INSERT died. sqlmock does no JSON validation so the earlier
+// tests were silent about it.
+//
+// Guard the exact Go-layer value we send by asserting the INSERT arg for
+// `payload` is "{}" (never ""). This is the same pattern as PR E's
+// EligibilitySnapshot.UnmetReasons guard.
+func TestReserve_EmptyPayloadDefaultsToEmptyJSONObject(t *testing.T) {
+	db, mock, cleanup := newLotteryTestDB(t)
+	defer cleanup()
+
+	// GORM omits granted_at from the INSERT column list because it has
+	// `<-:create;default:CURRENT_TIMESTAMP` — 7 args, not 8. Column order:
+	// external_ref, handler_type, user_id, activity_id, draw_id, amount, payload.
+	mock.ExpectBegin()
+	mock.ExpectExec("INSERT INTO `lottery_grant_ledger`").
+		WithArgs(
+			sqlmock.AnyArg(),           // external_ref
+			sqlmock.AnyArg(),           // handler_type
+			sqlmock.AnyArg(),           // user_id
+			sqlmock.AnyArg(),           // activity_id
+			sqlmock.AnyArg(),           // draw_id
+			sqlmock.AnyArg(),           // amount
+			payloadNotEmptyString{t},   // MUST be "{}", never ""
+		).
+		WillReturnResult(sqlmock.NewResult(1, 1))
+	mock.ExpectQuery("FROM `lottery_grant_ledger`").
+		WithArgs("lottery:100:200", 1).
+		WillReturnRows(sqlmock.NewRows([]string{"id", "external_ref"}).AddRow(int64(1), "lottery:100:200"))
+	mock.ExpectCommit()
+
+	svc := NewLedgerService()
+	err := db.Transaction(func(tx *gorm.DB) error {
+		// Intentionally leave Payload empty — the guard must default it.
+		_, _, e := svc.Reserve(context.Background(), tx, GrantLedger{
+			ExternalRef: "lottery:100:200",
+			HandlerType: "vpn_duration",
+			UserId:      42,
+			ActivityId:  100,
+			DrawId:      200,
+			Amount:      3,
+		})
+		return e
+	})
+	if err != nil {
+		t.Fatalf("Reserve: %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("expectations: %v", err)
+	}
+}
+
+// payloadNotEmptyString is a per-arg matcher: the value MUST be a non-empty
+// string; specifically "{}" per the PR F guard. Empty string is the exact F6
+// regression symptom (MySQL error 3140).
+type payloadNotEmptyString struct{ t *testing.T }
+
+func (m payloadNotEmptyString) Match(v driver.Value) bool {
+	s, ok := v.(string)
+	if !ok {
+		m.t.Fatalf("F6 guard: expected string for Payload, got %T (%v)", v, v)
+	}
+	if s == "" {
+		m.t.Fatalf("F6 regression: Payload must not be empty string (MySQL error 3140)")
+	}
+	if s != "{}" {
+		m.t.Fatalf("F6 guard: expected Payload==%q, got %q", "{}", s)
+	}
+	return true
+}