fix(plan): 修正补发触发条件与 backfilled 标记

三处缺陷：rescan 从未传 backfilled=True（字段形同虚设）；
loosened 谓词漏掉改 metric 和从下架转上架两种情况。
改为只要 metric/operator/threshold/visible 有变化就补发——
补发幂等，多跑一次只花一次扫描，漏跑是学生该得没得。
另补 rarity 的取值校验。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/superpowers/plans/2026-08-03-achievement-system.md

@@ -1310,7 +1310,10 @@ def rescan_achievement(achievement_id):
         if stat.user_id in already:
             continue
         try:
-            records = checker.unlock(stat.user, [achievement])
+            # backfilled=True：这是补发，不是学生刚刚挣到的。
+            # 前端据此只显示"已获得"而不显示具体日期——否则一次补发会给几百人
+            # 盖上同一个时间戳，把"最近获得"板块彻底冲垮
+            records = checker.unlock(stat.user, [achievement], backfilled=True)
             notify_achievements(stat.user_id, records)
         except Exception as e:
             logger.error(f"rescan_achievement failed for user {stat.user_id}: {e}")
@@ -1374,20 +1377,19 @@ class AchievementAdminAPI(APIView):
         if error:
             return self.error(error)
 
-        old_threshold = achievement.threshold
-        old_operator = achievement.operator
+        before = (achievement.metric, achievement.operator, achievement.threshold, achievement.visible)
         for field in ("name", "description", "icon", "rarity", "hidden", "metric", "operator", "threshold", "visible", "order"):
             if field in data:
                 setattr(achievement, field, data[field])
         achievement.save()
 
-        # 条件放宽（gte 调低阈值 / lte 调高阈值 / 换了比较符）时补发
-        loosened = (
-            achievement.operator != old_operator
-            or (achievement.operator == "gte" and achievement.threshold < old_threshold)
-            or (achievement.operator == "lte" and achievement.threshold > old_threshold)
-        )
-        if loosened and achievement.visible:
+        # 只要"谁能达成"这件事可能变了就补发，不去精细判断是否放宽。
+        # 补发是幂等的后台任务（unlock 用 get_or_create），多跑一次只花一次扫描；
+        # 漏跑却是学生已达标却拿不到，两个方向代价不对称。
+        # 早先的 loosened 谓词只看 operator/threshold，会漏掉两种情况：
+        # 换了 metric（换了维度）、以及从下架改成上架（草稿期已达标的人）。
+        after = (achievement.metric, achievement.operator, achievement.threshold, achievement.visible)
+        if achievement.visible and before != after:
             rescan_achievement.send(achievement.id)
         return self.success(_serialize(achievement))
 
@@ -1418,6 +1420,10 @@ def _validate(data):
         return "指标不存在"
     if data["operator"] not in ("gte", "lte"):
         return "比较符不合法"
+    # rarity 不校验的话，一个乱填的值会让 AchievementSummaryAPI 的四档统计
+    # 对不上：它按 Rarity.choices 遍历，野值算进总数却不出现在任何一档里
+    if data["rarity"] not in Rarity.values:
+        return "稀有度不合法"
     if not isinstance(data.get("threshold"), int):
         return "阈值必须是整数"
     return None