Add notification channel readiness and bounded batch tests

## Problem

The notification settings page can test DingTalk, email, and SMS one at a time, but operators cannot see which saved channels are actually ready or validate all configured channels in one bounded operation. This makes alert-delivery preparation repetitive and hides partial configuration.

## Proposed change

Add a notification readiness drawer that:

- summarizes saved DingTalk, email, and SMS configuration without exposing secrets;
- displays only webhook host names and recipient counts;
- tests only configured channels with a concurrency limit of two;
- isolates individual delivery failures and reports per-channel latency;
- keeps unconfigured channels visible and explicitly skipped;
- provides Chinese and English copy plus focused utility tests.

The workflow reuses the existing general-settings and notification-test endpoints and does not add a backend contract.

Start with the notification settings page and trace the existing general-settings and notification-test endpoints. Implement the readiness drawer with secret-safe summaries, a concurrency limit of two, per-channel failure and latency reporting, explicit skips, bilingual copy, and focused utility tests; done means configured channels are tested independently and unconfigured channels are visibly skipped.

Written by the indexing model from the issue text.