Admin Upsert Notification

Create or update a notification channel for a handle.

Body: { "handle": "brian@tandem-cc.com", # required "kind": "email", # required: email | slack | sms | webhook "target": "brian@acme.com", # required; shape depends on kind "enabled": true # optional (default true) }

Idempotent on (handle, kind, target): re-posting flips enabled if different.