feat(课堂看板): 「这节课」那排题号后面写上几点布置的，之后改题不变

class_lesson 加 created_at（0034，老数据用 updated_at 回填），「改」只动 updated_at；
清掉再布置算重新开始。

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

apps/web/src/oj/classroom/board.vue

@@ -485,6 +485,9 @@ function shortTitle(title: string, size: number) {
         <span v-for="problem in board.problems" :key="problem.problemId" class="chip">
           {{ problem.problemDisplayId }}
         </span>
+        <span v-if="board.assignedAt" class="assigned">
+          {{ parseTime(board.assignedAt, "HH:mm") }} 布置
+        </span>
         <n-button text class="edit" @click="startEdit">
           <template #icon><Icon icon="ph:pencil-simple" /></template>
           {{ board.source === "teacher" ? "改" : "布置" }}
@@ -854,6 +857,14 @@ function shortTitle(title: string, size: number) {
   font-weight: 600;
 }
 
+.assigned {
+  margin-left: 4px;
+  font-size: 14px;
+  font-variant-numeric: tabular-nums;
+  color: v-bind("theme.textColor3");
+  white-space: nowrap;
+}
+
 .edit {
   margin-left: 4px;
   font-size: 14px;