feat(lookup): 用户前缀查找放宽至单字符——中文姓氏单字搜索（全链路）

契约 minLength 2→1 + 服务端 bounds 1..64 + admin 两处/App 一处前端门槛
同步放宽；防抖测试基线更新（'s' 与合并后 'sta' 共两条）。

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

apps/admin/src/views/albums/detail/AlbumMembers.vue

@@ -175,7 +175,7 @@
   async function searchUsers(prefix: string) {
     query.value = prefix
     options.value = []
-    if (prefix.trim().length < 2) return
+    if (prefix.trim().length < 1) return
     searching.value = true
     try {
       const result = await fetchLookupUsers(prefix.trim())

apps/admin/src/views/albums/detail/AlbumTransferDialog.vue

@@ -112,7 +112,7 @@
   async function searchUsers(prefix: string) {
     query.value = prefix
     options.value = []
-    if (prefix.trim().length < 2) return
+    if (prefix.trim().length < 1) return
     searching.value = true
     try {
       const result = await fetchLookupUsers(prefix.trim())