抽出日期计算纯函数模块，并搭建后端测试基建

- 新增 server/src/occurrence.js：startOfDay/getNextOccurrence/daysBetween，today 可注入
- reminder.js 改为调用新模块，删除内联的 getThisYearDate/daysBetween
- 引入 node:test 作为测试框架，加 npm test 脚本
- 新增 test/helper.js（临时库辅助，供后续任务用）和 test/occurrence.test.js（7 个用例，覆盖农历跨年等边界）

server/test/helper.js

@@ -0,0 +1,13 @@
+const fs = require('fs')
+const os = require('os')
+const path = require('path')
+
+// 把 DB_PATH 指向临时目录，避免测试碰到真实库。
+// 必须在 require('../src/db') 之前调用，因为 db.js 在 require 时就读取 DB_PATH。
+function useTempDb() {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'bday-test-'))
+  process.env.DB_PATH = path.join(dir, 'test.db')
+  return dir
+}
+
+module.exports = { useTempDb }

server/test/occurrence.test.js

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+const test = require('node:test')
+const assert = require('node:assert')
+const { getNextOccurrence, daysBetween, startOfDay } = require('../src/occurrence')
+
+const fmt = (d) => `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
+
+test('公历纪念日：今年还没到就取今年', () => {
+  const anniv = { isLunar: false, solarMonth: 12, solarDay: 25 }
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2026, 7, 15))), '2026-12-25')
+})
+
+test('公历纪念日：今年已过就取明年', () => {
+  const anniv = { isLunar: false, solarMonth: 5, solarDay: 20 }
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2026, 7, 15))), '2027-05-20')
+})
+
+test('农历腊月十五：跨公历年不会算错一年', () => {
+  const anniv = { isLunar: true, lunarMonth: 12, lunarDay: 15, isLeapMonth: false }
+  // 2027-01-20 时，下一次应是 2 天后的 2027-01-22，而不是 2028 年
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2027, 0, 20))), '2027-01-22')
+  // 过了之后应跳到下一个农历年
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2027, 0, 23))), '2028-01-11')
+})
+
+test('农历四月十七', () => {
+  const anniv = { isLunar: true, lunarMonth: 4, lunarDay: 17, isLeapMonth: false }
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2026, 7, 15))), '2027-05-22')
+})
+
+test('isLunar 但缺农历字段时降级按公历算，不抛错', () => {
+  const anniv = { isLunar: true, lunarMonth: null, lunarDay: null, solarMonth: 6, solarDay: 5 }
+  assert.strictEqual(fmt(getNextOccurrence(anniv, new Date(2026, 7, 15))), '2027-06-05')
+})
+
+test('daysBetween 忽略时分秒', () => {
+  const today = new Date(2026, 7, 15, 23, 59, 0)
+  const target = new Date(2026, 7, 17, 0, 1, 0)
+  assert.strictEqual(daysBetween(target, today), 2)
+})
+
+test('startOfDay 归零时分秒', () => {
+  const d = startOfDay(new Date(2026, 7, 15, 13, 45, 30))
+  assert.strictEqual(d.getHours(), 0)
+  assert.strictEqual(d.getMinutes(), 0)
+})