Improve machines page: online status, formatted time, delete unbound placeholders.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/db.py

@@ -493,6 +493,24 @@ def delete_account(conn: sqlite3.Connection, account_id: int) -> None:
     conn.execute("DELETE FROM accounts WHERE id=?", (account_id,))
 
 
+def delete_machine(conn: sqlite3.Connection, machine_id: int) -> None:
+    """删除机器。在线中的不允许删；相关激活/成交仅解除 machine 引用。"""
+    m = conn.execute("SELECT * FROM machines WHERE id=?", (machine_id,)).fetchone()
+    if not m:
+        raise ValueError("机器不存在")
+    last = m["last_seen_at"]
+    online = False
+    try:
+        online = bool(last and (now() - float(last)) <= ONLINE_SECONDS)
+    except (TypeError, ValueError):
+        online = False
+    if online:
+        raise ValueError("机器当前在线，请先关闭客户端后再删")
+    conn.execute("UPDATE activations SET machine_id=NULL WHERE machine_id=?", (machine_id,))
+    conn.execute("UPDATE trades SET machine_id=NULL WHERE machine_id=?", (machine_id,))
+    conn.execute("DELETE FROM machines WHERE id=?", (machine_id,))
+
+
 def create_user(
     conn: sqlite3.Connection,
     username: str,