fix: 权限变更后不显示旧通知

- 新增 role_changed_at 字段，改权限时记录时间
- 通知查询用 MAX(注册时间, 权限变更时间) 过滤
- 确保新增权限的用户只看到变更之后的通知

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/database.py

@@ -163,6 +163,8 @@ def init_db():
         c.execute("ALTER TABLE users ADD COLUMN brand_bg TEXT")
     if "brand_align" not in user_cols:
         c.execute("ALTER TABLE users ADD COLUMN brand_align TEXT DEFAULT 'center'")
+    if "role_changed_at" not in user_cols:
+        c.execute("ALTER TABLE users ADD COLUMN role_changed_at TEXT")
 
     # Migration: add tags to user_diary
     diary_cols = [row[1] for row in c.execute("PRAGMA table_info(user_diary)").fetchall()]