重构日志与可观测性体系

新增单行文本编码器与结构化 GORM 日志，统一错误记录与请求日志策略，收紧日志文件权限并修复按天切分与压缩，支付回调参数脱敏，生产强制阿里云短信，RequestID 校验防注入，日志文案中文化。

backend/internal/middleware/request_id_test.go

@@ -0,0 +1,24 @@
+package middleware
+
+import "testing"
+
+func TestValidRequestID(t *testing.T) {
+	tests := []struct {
+		name  string
+		value string
+		want  bool
+	}{
+		{name: "标准 ID", value: "req-20260729_ab.cd", want: true},
+		{name: "空值", value: "", want: false},
+		{name: "包含空格", value: "bad id", want: false},
+		{name: "包含换行", value: "bad\nid", want: false},
+		{name: "超长", value: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", want: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := validRequestID(tt.value); got != tt.want {
+				t.Fatalf("validRequestID() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}