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/logic/common/promoEligibility_test.go

@@ -0,0 +1,46 @@
+package common
+
+import (
+	"testing"
+	"time"
+)
+
+func TestEvaluateInactiveUserExpire(t *testing.T) {
+	now := time.Date(2026, 5, 27, 8, 0, 0, 0, time.UTC)
+	params := promoRuleParams{InactiveMonths: 3}
+
+	tests := []struct {
+		name       string
+		lastExpire time.Time
+		want       bool
+	}{
+		{
+			name:       "permanent subscription is not inactive",
+			lastExpire: time.UnixMilli(0),
+			want:       false,
+		},
+		{
+			name:       "active subscription is not inactive",
+			lastExpire: now.Add(time.Hour),
+			want:       false,
+		},
+		{
+			name:       "expire at threshold is inactive",
+			lastExpire: now.AddDate(0, -3, 0),
+			want:       true,
+		},
+		{
+			name:       "expire before threshold is inactive",
+			lastExpire: now.AddDate(0, -3, -1),
+			want:       true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := evaluateInactiveUserExpire(tt.lastExpire, params, now); got != tt.want {
+				t.Fatalf("evaluateInactiveUserExpire() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}