fix(时区): 推送时间统一使用 UTC+8 而非服务器本地时区

Motivation:
推送时间原先依赖 time.Local，部署在不同时区的服务器会产生不一致的展示结果。统一为 UTC+8，确保所有订阅者看到的时间固定为北京时间。

Changes:

* 新增 internal/tz 时区工具，集中定义 UTC+8 与格式化逻辑
* 通知、crypto 策略、交易信号的时间渲染统一改用 UTC+8
* 补充测试断言 pushedAt 为 UTC+8 当前时间
* 更新 README 中关于注入时间的说明

Breaking Changes:
所有推送时间字段将按 UTC+8 展示，若服务部署在非 UTC+8 时区且依赖原本地时间行为，输出会发生变化。

internal/subscriber/cryptostrategy/convert.go

@@ -7,6 +7,8 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"aiaa-notification-service/internal/tz"
 )
 
 type envelope struct {
@@ -95,7 +97,7 @@ func Convert(body []byte) (string, map[string]interface{}, error) {
 	}
 	mergePayloadFields(data, payloadJSON)
 	if env.EventTime > 0 {
-		data["pushedAt"] = time.UnixMilli(env.EventTime).In(time.Local).Format("2006-01-02 15:04:05")
+		data["pushedAt"] = tz.Format(time.UnixMilli(env.EventTime), "2006-01-02 15:04:05")
 	}
 	if p.TotalGainTarget != 0 {
 		data["totalGainTarget"] = p.TotalGainTarget
@@ -260,7 +262,7 @@ func format(env envelope, p payload, action string) string {
 		lines = append(lines, fmt.Sprintf("策略: %s", p.StrategyCode))
 	}
 	if env.EventTime > 0 {
-		t := time.UnixMilli(env.EventTime).In(time.Local)
+		t := time.UnixMilli(env.EventTime).In(tz.CST)
 		lines = append(lines, fmt.Sprintf("Time: %s", t.Format("2006.01.02 15:04:05")))
 	}
 	return strings.Join(lines, "\n")

internal/subscriber/cryptostrategy/convert_test.go

@@ -50,6 +50,25 @@ func TestConvertParsesNestedPayload(t *testing.T) {
 	}
 }
 
+func TestConvertPushedAtIsUTC8(t *testing.T) {
+	body := []byte(`{
+		"eventType":"SIGNAL_RECEIVED","symbol":"CRV","direction":"LONG",
+		"payload":"{\"strategyCode\":\"ai-crypto-signals\",\"isClose\":true,\"price\":0.2528}",
+		"eventTime":1786899539730
+	}`)
+	_, data, err := Convert(body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if data["pushedAt"] != "2026-08-17 00:58:59" {
+		t.Fatalf("pushedAt=%v want UTC+8 2026-08-17 00:58:59", data["pushedAt"])
+	}
+	formatted, _ := data["formatted"].(string)
+	if !strings.Contains(formatted, "Time: 2026.08.17 00:58:59") {
+		t.Fatalf("formatted=%s", formatted)
+	}
+}
+
 func TestConvertCloseFlag(t *testing.T) {
 	body := []byte(`{
 		"eventType":"SIGNAL_RECEIVED","symbol":"BTC","direction":"SHORT",