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

Closes HIF-3 (Stage 1 P0 from QA smoke)

F4 (P0): EligibilitySnapshot.UnmetReasons=\"\" → MySQL error 3140 (Invalid JSON text: The document is empty)
- 修法：方式 A 对称守卫 UnmetReasons=\"[]\" (与 PrizeSnapshot.Config 的 \"{}\" 守卫模式一致)
- Stage 1 只有 passed=true 分支进 insertSnapshots，语义正确

F5 (P2 顺手): EvaluatedAt 显式 time.Now() 避免 GORM zero time 触 sql_mode STRICT

回归护栏：TestInsertSnapshots_UnmetReasonsIsValidJSON 用 sqlmock per-arg matcher (unmetReasonsNotEmpty + evaluatedAtNotZero)，退化时 t.Fatalf 立即抛出
CI 全绿；2 files, +108/-0；无 DB 变更

架构师侧透明：这是 PR C review 时漏检查——PrizeSnapshot.Config 的空串守卫看到了，但没同步检查 EligibilitySnapshot.UnmetReasons。single-code-path 的错觉是审查盲点，Stage 2 起会 sweep 所有 JSON/字符串写库字段。

internal/logic/lottery/draw/service.go

@@ -383,6 +383,14 @@ func (s *Service) insertSnapshots(ctx context.Context, tx *gorm.DB, draw *lotter
 		UserId:     draw.UserId,
 		ActivityId: draw.ActivityId,
 		Passed:     passedEligibility,
+		// UnmetReasons 是 JSON 列，MySQL 拒绝空字符串（error 3140）——
+		// Stage 1 只有 passed=true 进 insertSnapshots，语义上"没有未过项"，
+		// 用 "[]" 与 PrizeSnapshot.Config 的 "{}" 守卫对称。
+		// Stage 2 若开始持久化 passed=false 的失败评估，再改成真正的 marshal。
+		UnmetReasons: "[]",
+		// 显式 time.Now()：GORM 遇 zero time 有时会传 '0000-00-00 00:00:00'，
+		// 触 sql_mode STRICT。不依赖 DB DEFAULT CURRENT_TIMESTAMP。
+		EvaluatedAt: time.Now(),
 	}
 	return tx.WithContext(ctx).Create(&es).Error
 }

internal/logic/lottery/draw/service_test.go

@@ -3,6 +3,7 @@ package draw
 import (
 	"context"
 	"database/sql"
+	"database/sql/driver"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -511,3 +512,102 @@ func errAsCode(err error) (uint32, bool) {
 	}
 	return 0, false
 }
+
+// TestInsertSnapshots_UnmetReasonsIsValidJSON is the F4 regression guard.
+//
+// Before PR E, insertSnapshots created lottery_eligibility_snapshot rows with
+// UnmetReasons="" — MySQL error 3140 rejects empty strings on JSON columns,
+// so every real /draw request 100% failed the tx commit even though sqlmock
+// (which does no JSON validation) was happy. This test snapshots the exact
+// INSERT arg values and asserts:
+//   1. UnmetReasons must never be "" (it should be "[]")
+//   2. EvaluatedAt must not be the zero time.Time (STRICT sql_mode rejects
+//      '0000-00-00 00:00:00' on DATETIME NOT NULL)
+//
+// sqlmock cannot catch the JSON validity itself — only real MySQL can — but
+// it can catch the two upstream bugs that let bad values through the Go
+// layer. This is a defense-in-depth check.
+func TestInsertSnapshots_UnmetReasonsIsValidJSON(t *testing.T) {
+	db, mock, cleanup := newTestDB(t)
+	defer cleanup()
+
+	expectRunningActivity(mock, 100)
+	expectPrizePool(mock, 100, lottery.Prize{
+		Id: 1, ActivityId: 100, Slot: 0, Type: lottery.PrizeTypeNone, Name: "谢谢参与", Config: "{}", Weight: 100,
+	})
+
+	mock.ExpectBegin()
+	expectExistingDrawEmpty(mock)
+	mock.ExpectExec("INSERT INTO `lottery_draw`").
+		WillReturnResult(sqlmock.NewResult(555, 1))
+	mock.ExpectExec("INSERT INTO `lottery_prize_snapshot`").
+		WillReturnResult(sqlmock.NewResult(1, 1))
+
+	// Field order matches EligibilitySnapshot struct-tag order under GORM:
+	//   draw_id, user_id, activity_id, passed, unmet_reasons, evaluated_at.
+	// We assert UnmetReasons=="[]" (never "") and EvaluatedAt is non-zero.
+	mock.ExpectExec("INSERT INTO `lottery_eligibility_snapshot`").
+		WithArgs(
+			sqlmock.AnyArg(),         // draw_id
+			sqlmock.AnyArg(),         // user_id
+			sqlmock.AnyArg(),         // activity_id
+			sqlmock.AnyArg(),         // passed
+			unmetReasonsNotEmpty{t},  // MUST be "[]"
+			evaluatedAtNotZero{t},    // MUST be non-zero time
+		).
+		WillReturnResult(sqlmock.NewResult(1, 1))
+	mock.ExpectExec("UPDATE `lottery_draw`").
+		WillReturnResult(sqlmock.NewResult(0, 1))
+	mock.ExpectCommit()
+
+	svc := NewService(Deps{
+		DB:             db,
+		Enabled:        true,
+		Chance:         &fakeChance{consumeRemaining: 2},
+		Evaluator:      &fakeEvaluator{passed: true},
+		Picker:         &fakePicker{idx: 0},
+		Registry:       &stubRegistry{handlers: map[string]lottery.PrizeHandler{}},
+		ContextBuilder: fakeContextBuilder{},
+	})
+	_, err := svc.Draw(context.Background(), Request{UserId: 1, ActivityId: 100, ClientNonce: "f4-regression"})
+	if err != nil {
+		t.Fatalf("Draw: %v", err)
+	}
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Fatalf("expectations: %v", err)
+	}
+}
+
+// unmetReasonsNotEmpty is a per-arg matcher: the value MUST be the string
+// "[]"; the empty string is the exact F4 regression we are guarding against.
+type unmetReasonsNotEmpty struct{ t *testing.T }
+
+func (m unmetReasonsNotEmpty) Match(v driver.Value) bool {
+	s, ok := v.(string)
+	if !ok {
+		m.t.Fatalf("F4 guard: expected string for UnmetReasons, got %T (%v)", v, v)
+	}
+	if s == "" {
+		m.t.Fatalf("F4 regression: UnmetReasons must not be empty string (MySQL error 3140)")
+	}
+	if s != "[]" {
+		m.t.Fatalf("F4 guard: expected UnmetReasons==%q, got %q", "[]", s)
+	}
+	return true
+}
+
+// evaluatedAtNotZero is a per-arg matcher: the value MUST be a non-zero
+// time.Time; the zero time is the F5 regression that STRICT sql_mode rejects
+// as '0000-00-00 00:00:00'.
+type evaluatedAtNotZero struct{ t *testing.T }
+
+func (m evaluatedAtNotZero) Match(v driver.Value) bool {
+	tv, ok := v.(time.Time)
+	if !ok {
+		m.t.Fatalf("F5 guard: expected time.Time for EvaluatedAt, got %T (%v)", v, v)
+	}
+	if tv.IsZero() {
+		m.t.Fatalf("F5 regression: EvaluatedAt must not be zero time")
+	}
+	return true
+}