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/src/reminder.js

@@ -2,6 +2,7 @@ const cron = require('node-cron')
 const db = require('./db')
 const wx = require('./wx')
 const lunar = require('./lunar')
+const occurrence = require('./occurrence')
 
 const TEMPLATE_ID = process.env.WX_TEMPLATE_ID
 const MINIPROGRAM_STATE = process.env.WX_MINIPROGRAM_STATE || 'formal'
@@ -27,40 +28,6 @@ function formatDate(date) {
   return `${y}年${m}月${d}日`
 }
 
-// 算出"距离今年（或明年）这个纪念日还有多少天"
-// 农历纪念日按 lunarMonth/lunarDay 计算每年对应的公历日期，否则按 solarMonth/solarDay
-function getThisYearDate(anniv) {
-  const today = new Date()
-  today.setHours(0, 0, 0, 0)
-
-  if (anniv.isLunar && anniv.lunarMonth && anniv.lunarDay) {
-    // 闰月生日按 A 方案处理：若当年无对应闰月，lunarToSolar 内部已自动按普通月份算
-    const todayLunar = lunar.solarToLunar(today)
-    const wantLeap = !!anniv.isLeapMonth
-    let target = lunar.lunarToSolar(todayLunar.year, anniv.lunarMonth, anniv.lunarDay, wantLeap)
-    target.setHours(0, 0, 0, 0)
-    if (target < today) {
-      target = lunar.lunarToSolar(todayLunar.year + 1, anniv.lunarMonth, anniv.lunarDay, wantLeap)
-      target.setHours(0, 0, 0, 0)
-    }
-    return target
-  }
-
-  let target = new Date(today.getFullYear(), anniv.solarMonth - 1, anniv.solarDay)
-  target.setHours(0, 0, 0, 0)
-  if (target < today) {
-    target = new Date(today.getFullYear() + 1, anniv.solarMonth - 1, anniv.solarDay)
-    target.setHours(0, 0, 0, 0)
-  }
-  return target
-}
-
-function daysBetween(target) {
-  const today = new Date()
-  today.setHours(0, 0, 0, 0)
-  return Math.round((target - today) / 86400000)
-}
-
 // 检查今天是否已经给这条纪念日发过提醒
 function alreadySentToday(anniversaryId) {
   const start = new Date()
@@ -87,8 +54,8 @@ async function runOnce() {
 
   for (const anniv of list) {
     try {
-      const target = getThisYearDate(anniv)
-      const daysUntil = daysBetween(target)
+      const target = occurrence.getNextOccurrence(anniv)
+      const daysUntil = occurrence.daysBetween(target)
 
       const shouldRemind = (daysUntil === 0) || (daysUntil === (anniv.remindDays || 0))
       if (!shouldRemind) continue