fix(班级筛选): 输入 ks251 会把 2511 班的学生也算进来

班级号 3~4 位并存，按用户名前缀/包含匹配时 ks251 会捞到 ks2510~ks2519。
新增 classPrefixCondition：ks + 3 位以上数字按具体班级匹配（后面不能再跟数字），
1~2 位仍是年级前缀。后台首页的用户排名、随机抽签，以及题单的学生进度筛选
（只填数字时补上 ks）都改用它。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/routes/admin/account.ts

@@ -34,7 +34,7 @@ import { revokeUserSessions } from "../../auth/session"
 import { requireSuperAdmin, type AppEnv } from "../../auth/middleware"
 import { db, schema } from "../../db"
 import { failure, parseBody, success } from "../../http"
-import { queryInteger, sampleUser } from "../helpers"
+import { classPrefixCondition, queryInteger, sampleUser } from "../helpers"
 
 export const adminAccountRoutes = new Hono<AppEnv>()
 
@@ -130,7 +130,11 @@ adminAccountRoutes.get("/rankings/users", requireSuperAdmin, async (c) => {
   const where = and(
     inArray(schema.user.adminType, [...STUDENT_ROLES]),
     eq(schema.user.isDisabled, false),
-    keyword ? ilike(schema.user.username, `%${keyword}%`) : undefined,
+    // 输入班级前缀（ks251）按班级匹配，否则照旧按用户名包含
+    keyword
+      ? (classPrefixCondition(keyword) ??
+          ilike(schema.user.username, `%${keyword}%`))
+      : undefined,
   )
 
   const [totalRows, rows] = await Promise.all([