[Admin Console] Phase 7 : Admin API for notifications (config, test delivery, history)

[Ferryx349]

Parents : #637, #759

Summary

Authenticated admin endpoints so operators can manage notification targets and inspect delivery history without editing YAML.

Scope

• GET /admin/notifications :- config (secrets masked)
• PATCH /admin/notifications :- targets, per-event toggles, retry policy (reuse admin settings validation patterns)
• POST /admin/notifications/test :- send "Test notification from <relay name>" to a target
• GET /admin/notifications/deliveries :- paginated history (filter by status / event type)
• Auth, rate limits, JSON body limits :- same as existing admin routes
• Unit tests in test/unit/routes/admin.spec.ts