feat(achievement): 添加用户侧成就 API

新增 achievements 列表、summary、pending 三个用户侧端点，隐藏未解锁成就
在序列化层打码（name/description/icon 替换为占位符，不下发 progress）。

achievement/urls/oj.py

@@ -0,0 +1,9 @@
+from django.urls import path
+
+from achievement.views.oj import AchievementListAPI, AchievementPendingAPI, AchievementSummaryAPI
+
+urlpatterns = [
+    path("achievements", AchievementListAPI.as_view(), name="achievement_list_api"),
+    path("achievements/summary", AchievementSummaryAPI.as_view(), name="achievement_summary_api"),
+    path("achievements/pending", AchievementPendingAPI.as_view(), name="achievement_pending_api"),
+]