Admin Console] Phase 7: Admin Console notifications page (targets, toggles, test, log)

[Ferryx349]

Parent: #637, #759,#760

Summary

Operator-facing UI with configure alerts, test delivery, and view delivery history.

Scope

• New Notifications section in admin console nav
• Add/edit/remove targets (HTTP, Discord, Slack, Telegram)
• Per-event toggles: invoice created / paid / failed, settings changed, relay restart
• Retry policy (simple fields or collapsible “Advanced”)
• Test delivery button per target -> calls 7B test endpoint, show success/error inline
• Delivery history table (time, event, target, status, error snippet) with pagination
• Match existing admin dashboard styling and a11y patterns