feat(hub): decouple from users and add global alerts

[svenvg93]

📃 Description

Decouples alerts from users and adds global alerts. Alerts are now a single shared record per system/type instead of one per user. Admins can configure global alert defaults that automatically sync to all systems, with per-system exclusion support.

Closes #1914
Closes #897

🪵 Changelog

➕ Added

• Global Alerts settings page (admin-only) — alert defaults that auto-apply to all systems
• "Global" badge with tooltip on per-system alerts managed by a global alert
• Per-system exclusions per global alert type
• Users can enable notifications if needed and subscribe to specific systems.

✏️ Changed

• Alerts are now shared across users — one record per system/type
• "Copy from" skips globally managed alerts on the target system

🔧 Fixed

• excluded_systems changed from relation to JSON field to avoid validation errors when an admin is not a member of all systems

🗑️ Removed

• user field from alerts and alerts_history collections
• "All Systems" tab from the alerts sheet

📷 Screenshots