feat(leaderboard): 训练完强刷 + 下拉刷新 force

- 云函数 _fetchLatestByOpenid 支持 force 跳过实例缓存重新扫描
- 训练完成设 _lb_force_refresh 标志,下次打开排行榜强刷,新数据立即可见
- 下拉刷新带 force=true,拉取最新数据
- 平时常规打开仍走 5min 缓存 + 客户端乐观缓存,不增加无谓扫描

Co-Authored-By: Claude <noreply@anthropic.com>

cloudfunctions/leaderboard/index.js

@@ -43,11 +43,11 @@ const tsOf = (d) => {
 // per openid. Uses cursor pagination (_id > lastId) instead of skip(N):
 // skip is O(N) and degrades as the collection grows; a cursor is constant
 // cost per page, and default get() order (_id ascending) won't drop/dup.
-const _fetchLatestByOpenid = async () => {
-  if (_docsCache && (Date.now() - _docsCache.ts) < CACHE_TTL) {
+const _fetchLatestByOpenid = async (force) => {
+  if (!force && _docsCache && (Date.now() - _docsCache.ts) < CACHE_TTL) {
     return _docsCache.latestByOpenid
   }
-  // 缓存过期:复用 in-flight promise,避免并发请求各自全表扫描(缓存击穿)
+  // 缓存过期或强刷(force):复用 in-flight promise,避免并发请求各自全表扫描(缓存击穿)
   if (_docsCachePromise) return _docsCachePromise
   _docsCachePromise = (async () => {
   const latestByOpenid = new Map()
@@ -81,7 +81,7 @@ const _fetchLatestByOpenid = async () => {
 }
 
 exports.main = async (event) => {
-  const { period, maxRank } = event || {}
+  const { period, maxRank, force } = event || {}
   const limit = Math.max(1, Math.min(parseInt(maxRank) || DEFAULT_MAX_RANK, 500))
   if (!period) return { err: 'missing period' }
 
@@ -107,7 +107,7 @@ exports.main = async (event) => {
   // start add() bug; only the latest updatedAt counts, else we'd sum the
   // same records N times and inflate the board. Latest (not first) also
   // gives the freshest profile.nickname before admin-dedupe runs.
-  const latestByOpenid = await _fetchLatestByOpenid()
+  const latestByOpenid = await _fetchLatestByOpenid(force)
 
   // Second pass: each openid appears exactly once, so the accumulation
   // logic doesn't need any dedup guards.