feat(servers): 凭据轮询登录与连接失败列表

- 密码/SSH密钥预设增加用户名；快速添加(IP)轮询全部预设尝试 SSH 登录
- 成功入 servers 列表；失败写入 pending_servers 并支持重试/删除
- 新增 credential_poller、try_ssh_login 与 add-by-ip/pending API

server/infrastructure/ssh/asyncssh_pool.py

@@ -25,6 +25,51 @@ from server.infrastructure.database.crypto import decrypt_value
 logger = logging.getLogger("nexus.asyncssh_pool")
 
 
+async def try_ssh_login(
+    host: str,
+    port: int,
+    username: str,
+    *,
+    password: str | None = None,
+    private_key: str | None = None,
+    timeout: float = 8.0,
+) -> tuple[bool, str]:
+    """One-shot SSH login probe (not pooled). Returns (ok, error_message)."""
+    from server.config import settings
+
+    if not password and not private_key:
+        return False, "未提供密码或私钥"
+
+    connect_kwargs: dict = {
+        "host": host,
+        "port": port or 22,
+        "username": username or "root",
+        "connect_timeout": timeout,
+    }
+    if settings.SSH_STRICT_HOST_CHECKING.lower() in ("false", "0", "no"):
+        connect_kwargs["known_hosts"] = None
+
+    if password:
+        connect_kwargs["password"] = password
+    elif private_key:
+        connect_kwargs["client_keys"] = [asyncssh.import_private_key(private_key)]
+
+    conn: asyncssh.SSHClientConnection | None = None
+    try:
+        conn = await asyncssh.connect(**connect_kwargs)
+        return True, ""
+    except asyncssh.Error as e:
+        return False, str(e)[:200]
+    except Exception as e:
+        return False, str(e)[:200]
+    finally:
+        if conn is not None:
+            try:
+                conn.close()
+            except Exception:  # noqa: S110 — best-effort cleanup
+                pass
+
+
 @dataclass
 class PooledConnection:
     """A tracked asyncssh connection with reference counting"""