Add per-monitor notification channel scoping

[greyair]

Summary

Allow Custom Webhook and Telegram notification channels to optionally target specific monitors.

When no monitors are selected, behavior remains unchanged and the channel receives events for all monitors.

Changes

• add optional monitor_ids to notification channel configuration
• filter monitor, incident, and maintenance events by configured monitor scope
• keep test.ping unscoped so channel connectivity can always be tested
• add monitor selection UI to the notification channel form
• add English / Simplified Chinese / Traditional Chinese labels
• add dispatch coverage for out-of-scope monitor events

Test Plan

• pnpm lint passes
• pnpm typecheck passes
• pnpm test passes
• Manually tested in a deployed fork: scoped channels only receive matching monitor events; empty selection retains existing all-monitor behavior

Related Issues

N/A