Add centralized “Notifications” section to manage reusable channels

[gorkem-bwl]

Why this matters

Right now every monitor stores its own notification settings. Changing a webhook or email means hunting through each monitor, even with bulk edit. It’s slow, error-prone, and feels tacked on for a monitoring tool that lives on alerts.

What needs to be fixed

• Managing notifications per monitor is painful when you’ve got dozens.
• Bulk edit helps a bit but still forces you to touch multiple monitors.
• A simple change, like pointing a Discord webhook to a new channel, turns into busywork.

What we need instead

• A “Notifications” section in the sidebar next to Uptime, Pagespeed, Infra.
• Users create reusable notification channels (Discord, Slack, Email, generic Webhook, etc.).
• Monitors reference those channels instead of duplicating settings.
• Updating one notification channel instantly applies to every monitor using it.