Add log4js fileSync logger module and formatError helper

src/server/log-format.test.ts

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+import { describe, it, expect } from "vitest";
+import { formatError } from "./log-format";
+
+describe("formatError", () => {
+  it("returns stack for Error", () => {
+    const e = new Error("boom");
+    expect(formatError(e)).toBe(e.stack);
+  });
+  it("falls back to message when stack missing", () => {
+    const e = new Error("boom");
+    e.stack = undefined as unknown as string;
+    expect(formatError(e)).toBe("boom");
+  });
+  it("stringifies non-Error primitives", () => {
+    expect(formatError("oops")).toBe("oops");
+    expect(formatError(42)).toBe("42");
+  });
+  it("handles null / undefined", () => {
+    expect(formatError(null)).toBe("Unknown error");
+    expect(formatError(undefined)).toBe("Unknown error");
+  });
+});

src/server/log-format.ts

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+/**
+ * 把任意错误/值格式化为带堆栈的字符串，用于日志记录。
+ * - Error：优先 stack，缺则 message
+ * - null/undefined：'Unknown error'
+ * - 其它：String(value)
+ */
+export function formatError(err: unknown): string {
+  if (err instanceof Error) return err.stack || err.message;
+  if (err === null || err === undefined) return "Unknown error";
+  return String(err);
+}

src/server/logger.ts

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+import log4js from "log4js";
+
+// fileSync = 同步写：进程 OOM/被杀前日志已落盘，满足崩溃诊断刚需。
+log4js.configure({
+  appenders: {
+    app: {
+      type: "fileSync",
+      filename: "logs/app/app.log",
+      maxLogSize: 10 * 1024 * 1024, // 10MB
+      backups: 5,
+      layout: { type: "pattern", pattern: "[%d{ISO8601}] [%p] [%c] %m" },
+    },
+  },
+  categories: { default: { appenders: ["app"], level: "info" } },
+});
+
+export { formatError } from "./log-format";
+
+export const apiLogger = log4js.getLogger("api");
+export const exportLogger = log4js.getLogger("export");