[Feature] Multi-Channel Notification Dispatch

[andreicscs]

Problem: Operators currently can only configure a single alert channel.
Solution: Expand the notification_channels table to support multiple active destinations (e.g., Slack + Ntfy simultaneously) allowing to select which severities hit which channel. Update the notify.Service to concurrently broadcast notifications using isolated per-channel retries. This allows better personalization and integration of honeywire with existing environments.