修复(#75): 按数量精确匹配套餐列表促销

- loadSubscribePromoMap 返回 map[subscribeID]map[quantity]*SubscribePromo 二级映射
- 候选查询按 subscribe_id、quantity、priority DESC 排序，一次取出所有 quantity 档位
- 下单路径 QueryEligibleRules 将 quantity 条件移至 JOIN，精确匹配
- 永久订阅(expire_time=0)用 CASE WHEN 排序兜底，不再误判为回归促销
- 新增 promoEligibility、model、subscribe promo 单元测试

Co-authored-by: multica-agent <github@multica.ai>

internal/model/promo/model_test.go

@@ -0,0 +1,39 @@
+package promo
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+)
+
+func TestQueryEligibleRulesFiltersByQuantity(t *testing.T) {
+	db, err := gorm.Open(mysql.New(mysql.Config{
+		DSN:                       "gorm:gorm@tcp(localhost:9910)/gorm?charset=utf8&parseTime=True&loc=Local",
+		SkipInitializeWithVersion: true,
+	}), &gorm.Config{DryRun: true, DisableAutomaticPing: true})
+	if err != nil {
+		t.Fatalf("open dry-run db: %v", err)
+	}
+
+	model := &defaultPromoModel{db: db}
+	var list []*RuleWithPrice
+	tx := model.eligibleRulesQuery(context.Background(), 11, 3).Find(&list)
+	stmt := tx.Statement
+	sql := stmt.SQL.String()
+	if !strings.Contains(sql, "JOIN subscribe_promo AS sp ON sp.promo_rule_id = pr.id AND sp.quantity = ?") {
+		t.Fatalf("SQL missing quantity join condition: %s", sql)
+	}
+
+	if len(stmt.Vars) < 2 {
+		t.Fatalf("SQL vars length = %d, want at least 2; vars=%v", len(stmt.Vars), stmt.Vars)
+	}
+	if got, want := stmt.Vars[0], int64(3); got != want {
+		t.Fatalf("first SQL var = %v, want quantity %d; vars=%v", got, want, stmt.Vars)
+	}
+	if got, want := stmt.Vars[1], int64(11); got != want {
+		t.Fatalf("second SQL var = %v, want subscribe_id %d; vars=%v", got, want, stmt.Vars)
+	}
+}