Custom domain orphaned after failed add + parent app deletion: every re-add fails with unknown error (legal.greaterbergen.org)

### Describe the bug

Custom domain `legal.greaterbergen.org` cannot be added to any Static Web App. Every attempt fails with:

> An unknown error has occurred while adding your custom domain. Please try again later.

and the failed `customDomains` child resource then hangs in `Deleting` state for many hours.

The hostname appears to be orphaned in the SWA backend: the very first add attempt (2026-08-11, app `gbca-legal` in `gbca-web-rg`, East US 2) failed with the same unknown error, its delete hung, and the parent app was deleted while the child was still `Deleting`. Since then, every add of this hostname on a freshly created app fails identically. Requesting release of the stale hostname registration for `legal.greaterbergen.org`.

### Evidence that client-side config is correct

- CNAME `legal.greaterbergen.org` -> `delightful-glacier-00a9cda0f.7.azurestaticapps.net` verified on the authoritative name servers (GoDaddy ns07/ns08.domaincontrol.com) and public resolvers (8.8.8.8, 1.1.1.1) before each attempt
- No CAA records on the zone, no DNSSEC (no DS record), no conflicting A/TXT records
- Two sibling subdomains on the SAME zone with identical CNAME configuration (`hsrp.greaterbergen.org`, `unityday.greaterbergen.org`) attached to sibling SWAs without issue
- Both `cname-delegation` and `dns-txt-token` validation methods fail the same way (`dns-txt-token` never returns a validation token; the record goes straight to `Deleting`)
- Reproduced across two different app instances (original app deleted; current app `gbca-legal`, default hostname `delightful-glacier-00a9cda0f.7.azurestaticapps.net`)

### Current state

- App: `gbca-legal`, resource group `gbca-web-rg`, East US 2, Free tier
- `az staticwebapp hostname list` shows the domain stuck in `Deleting` with errorMessage "An unknown error has occurred while adding your custom domain."
- Retried roughly every 30 minutes for 14+ hours; no change

### What we are asking

Please locate and release the orphaned/stale backend registration for `legal.greaterbergen.org` so it can be added to app `gbca-legal`. Happy to provide subscription/correlation IDs privately if needed.

Start by reproducing the failure with `az staticwebapp hostname list` for `gbca-legal` and compare the `cname-delegation` and `dns-txt-token` attempts described in the issue. Done means the orphaned registration is released and `legal.greaterbergen.org` can be added successfully; no repository file or test entry point is identified, so this requires Azure backend investigation.

Written by the indexing model from the issue text.