Static Web App cannot be deleted — orphaned linkedBackend is readable but returns 404 on DELETE/unlink

I have a Standard-tier Static Web App that cannot be deleted because of an orphaned `linkedBackend` record. The link points to a Web App that no longer exists, and the record is readable but unaddressable by the write/delete path. This has been blocking subscription cancellation for months.

**Resource**

- Subscription: ``
- Resource: `/subscriptions//resourceGroups/formflow-rg/providers/Microsoft.Web/staticSites/formflow-app-prod`
- SKU: Standard, Region: eastus2
- Only resource in the subscription

**Symptom**

Both direct DELETE on the SWA and resource-group cascade DELETE fail with the opaque error:

```json
{"code":"ResourceOperationFailure",
"message":"The resource operation completed with terminal provisioning state 'Failed'.",
"details":[{"code":"Unspecified","message":""}]}
```

**Root cause visible from ARM**

The site has a build named `preview` holding a `linkedBackends` record `backend1` pointing at:

```
/subscriptions//resourceGroups/formflow-rg/providers/Microsoft.Web/sites/formflow-backend-104793
```

That target Web App no longer exists (confirmed via `az resource list` across all my subscriptions). The link is dangling.

The metadata is in an inconsistent state:

- `GET .../builds/preview/linkedBackends` returns `backend1` with `provisioningState: Succeeded`
- `DELETE .../builds/preview/linkedBackends/backend1` → **404 NotFound** on api-versions `2024-04-01`, `2023-12-01`, `2023-01-01`, `2022-09-01`, `2022-03-01`, `2021-03-01`
- `POST .../linkedBackends/backend1/unlink` and `.../builds/preview/linkedBackends/backend1/unlink` → **404 NotFound / BadRequest**

So the record exists from the read path but is unaddressable from the write path. Only the platform can clear it.

**Confirmation the linked backend is what's blocking everything**

Attempting to downgrade the SKU to Free returns a clear error pointing at the same record:

```
Sku is invalid. Free SKU is invalid. Linked Backends are not allowed in this SKU.
The following builds have linked backends configured: preview
```

The downgrade path enumerates `backend1` correctly; only the unlink/delete path cannot remove it. This is consistent with corruption in the `linkedBackends` storage on the RP side.

**Activity-log entries**

```
2026-04-28T13:56:11Z Microsoft.Web/staticSites/linkedBackends/unlink/action Failed/NotFound → {"error":{"code":"BadRequest","message":""}}
2026-04-28T13:56:11Z Microsoft.Web/staticSites/builds/linkedBackends/unlink/action Failed/NotFound → {"error":{"code":"BadRequest","message":""}}
2026-04-28T13:56:10Z Microsoft.Web/staticSites/builds/linkedBackends/delete Failed/NotFound → "Resource Not Found"
2026-04-28T14:01:24Z Microsoft.Web/staticSites/builds/delete Failed → Unspecified
```

**What I have tried**

- DELETE on the Static Web App directly — `Unspecified`
- DELETE on resource group `formflow-rg` — `Unspecified` on `staticSites/builds/delete`
- DELETE on `linkedBackends/backend1` across 6 API versions — all 404
- POST `unlink` action, both site- and build-scoped — 404 / BadRequest
- PATCH SKU `Standard` → `Free` — blocked by the dangling backend
- Subscription cancel via `Microsoft.Subscription/cancel` — blocked: `ResourcesExistingOnSubscription`
- Billing-API cancel via `Microsoft.Billing/.../billingSubscriptions/.../cancel` — not supported for Individual MCA
- DELETE `builds/preview` directly — accepted, but the cascading flow fails on the same record

**Questions**

1. Is there a supported way to force-remove a dangling `linkedBackend` whose target resource no longer exists, when both `DELETE` and the `unlink` action return 404 on the record itself?
2. If not, can someone from the Static Web Apps RP / Engineering team clear the `backend1` record on build `preview` of `formflow-app-prod` server-side so the SWA can be deleted and the subscription closed?