feat: refine task and countdown interactions

backend/mvp.py

@@ -827,6 +827,19 @@ async def pin_countdown(countdown_id: UUID, user: User = Depends(current_user),
     return countdown_dict(row)
 
 
+@router.delete("/countdowns/{countdown_id}/pin")
+async def unpin_countdown(countdown_id: UUID, user: User = Depends(current_user), db: AsyncSession = Depends(get_db)):
+    row = await owned_countdown(db, user.id, countdown_id)
+    if row.archived_at is not None:
+        raise HTTPException(409, "已归档倒数日不能取消置顶")
+    row.pinned = False
+    row.updated_at = utcnow()
+    audit(db, user.id, "update", "countdown", row.id)
+    await db.commit()
+    await db.refresh(row)
+    return countdown_dict(row)
+
+
 @router.delete("/countdowns/{countdown_id}", status_code=204)
 async def archive_countdown(countdown_id: UUID, user: User = Depends(current_user), db: AsyncSession = Depends(get_db)):
     row = await owned_countdown(db, user.id, countdown_id)