Feature Request: Ability to bulk-assign Notification Channels to Alerts

Currently, after creating a new notification channel (e.g., the Email channel), there is no way to apply this channel to multiple alerts at once. Manually editing, selecting the channel, and saving every individual alert is extremely time-consuming and prone to human error when managing a large number of monitors.

Proposed Solutions:

1. Global Defaults: An option in the Notification Channel settings to "Enable for all current and future alerts."
2. Bulk Edit Mode: A checkbox selection on the main Dashboard/Alerts list to "Add Notification Channel" to all selected items.
3. Default Channel: The ability to mark a specific channel as the 'Default' so it is automatically attached to any new alert created.

Start at the Dashboard/Alerts list, Notification Channel settings, and new-alert creation flow described in the issue, then trace how alerts and channels are currently connected. Before coding, confirm whether the intended scope is global defaults, bulk edit, or default channel; done should include an agreed behavior and coverage for the selected flow.

Written by the indexing model from the issue text.