[V2] Public Admin API reference for the v2 surface

[perki]

Documentation pass for the v2 admin HTTP surface and operator CLIs. Replaces the v1 admin reference (which still describes the retired config-leader/follower architecture).

Covers:

• /system/admin/* HTTP routes: platform-config CRUD, users, cores, certs, DNS records, mail templates, observability, multi-core bootstrap, backup, migration runners, audit log.
• Authentication: bearer token (auth.adminAccessKey), admin-user session with MFA.
• Service-info contract: the adminPanel field advertising compatible web-app version letters, available capabilities, and operator lockdown.
• Operator CLIs as the underlying contract: bin/install.js (greenfield install), bin/bootstrap.js (multi-core onboarding), bin/backup.js, bin/migrate.js, bin/dns-records.js, bin/observability.js.
• Lockdown profile catalog (Dokku, GitOps, Cloudflare-fronted, air-gap) operators can copy-paste.

Published on the dev-site reference layout for both human reading and OpenAPI-driven SDK generation.