feat(通知模板): 支持缺字段按空值渲染与空行省略

Motivation:
统一 crypto-strategy 与 trade-signal 两类交易信号的模板字段，使同一套通知模板可复用；字段缺失或为空时不再导致渲染报错或输出空行，通知内容更整洁。

Changes:

* 渲染器缺失字段改为按空值处理，新增 line 函数实现空值整行省略
* crypto-strategy 信号补充止盈价、止损价、平均价、收益额等模板字段
* trade-signal 信号保留原始报文中的额外字段以适配统一模板
* 修复结构化日志将 error 作为值直接输出导致的格式问题

internal/subscriber/handle.go

@@ -70,6 +70,13 @@ type MessageConverter interface {
 	Convert(body []byte) (event string, data map[string]interface{}, err error)
 }
 
+func errText(err error) string {
+	if err == nil {
+		return ""
+	}
+	return err.Error()
+}
+
 func HandleMessage(ctx context.Context, in HandleInput, conv MessageConverter, lookup SourceLookup, process ProcessFunc) Disposition {
 	owned := false
 	hash := ""
@@ -77,7 +84,7 @@ func HandleMessage(ctx context.Context, in HandleInput, conv MessageConverter, l
 		hash = MessageHash(in.Body)
 		ok, err := in.Deduper.Claim(ctx, hash)
 		if err != nil {
-			slog.Warn("dedup claim failed, processing anyway", "hash", hash, "error", err)
+			slog.Warn("dedup claim failed, processing anyway", "hash", hash, "error", errText(err))
 		} else if !ok {
 			slog.Info("duplicate message, ack", "hash", hash)
 			return DispositionAck
@@ -94,13 +101,13 @@ func HandleMessage(ctx context.Context, in HandleInput, conv MessageConverter, l
 
 	event, data, err := conv.Convert(in.Body)
 	if err != nil {
-		slog.Warn("invalid signal, ack", "hash", hash, "raw", raw, "error", err)
+		slog.Warn("invalid signal, ack", "hash", hash, "raw", raw, "error", errText(err))
 		return DispositionAck
 	}
 
 	src, err := lookup(ctx, in.SourceName)
 	if err != nil || src == nil || src.Status != 1 {
-		slog.Warn("source unavailable, ack", "source", in.SourceName, "hash", hash, "raw", raw, "error", err)
+		slog.Warn("source unavailable, ack", "source", in.SourceName, "hash", hash, "raw", raw, "error", errText(err))
 		return DispositionAck
 	}
 
@@ -116,14 +123,14 @@ func HandleMessage(ctx context.Context, in HandleInput, conv MessageConverter, l
 		return DispositionAck
 	}
 	if errors.Is(err, notify.ErrUnprocessable) {
-		slog.Warn("unprocessable notify, ack", "source", src.Name, "event", event, "hash", hash, "raw", raw, "error", err)
+		slog.Warn("unprocessable notify, ack", "source", src.Name, "event", event, "hash", hash, "raw", raw, "error", errText(err))
 		return DispositionAck
 	}
 
 	disp := DecideRetry(RetryCount(in.Headers), in.MaxRetry)
 	if owned && in.Deduper != nil {
 		if relErr := in.Deduper.Release(ctx, hash); relErr != nil {
-			slog.Warn("dedup release failed", "hash", hash, "error", relErr)
+			slog.Warn("dedup release failed", "hash", hash, "error", errText(relErr))
 		}
 	}
 	return disp