Atelier: validate the agency placement pin against live servers (ranta is gone, all-zero id is not 'master')

Imported from GitHub issue El-SaMa/oma#138 by @El-SaMa.

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reseller_accounts.app_server_id pins an agency's new sites to one app node. Two problems found 2026-10-01:

1. Deleted servers are accepted. QA agencies were pinned to ranta (933bd0fb-…), which no longer exists in the hosting panel, so provisioning failed with 404: server (agencies see the "capacity" reason).
2. The all-zero id does not target the master. The panel treats 00000000-0000-0000-0000-000000000000 as "no preference" and auto-places the site (a QA site landed on ns1).

What

• In staffUpdateResellerAccount (services/core-api/src/reseller/account.ts) / oma.admin_reseller_account, check appServerId against the live server list (the same source as oma.admin_hosting_servers) and refuse unknown ids with a clear error. Refuse the all-zero id, or document it as auto-placement.
• Optionally show a warning in /admin/resellers when an agency is pinned to a server that no longer exists.
• Update docs that still list ranta as an app node: CLAUDE.md access map and the web-A rule, and the handbook.

Done when

A pin to a non-existent server is refused, the all-zero id is handled explicitly, and docs no longer list ranta as a live node.