chore(backend): remove retired enhanced email deliverability setting

[jescalan]

Summary

Follow-up to the Postmark send-path removal (clerk_go#21398). That PR deleted the behavior; this one deletes the field.

The setting no longer affects anything: routing, unverified-DNS suppression, template forcing, and the magic-link validator were all removed, and every remaining instance was flipped to false in production on 2026-08-26 (20 production + 14 development). What is left is an inert boolean on the API surface, plus dead config.

Rollout ordering (important)

This removes enhanced_email_deliverability from the FAPI /v1/environment payload, where it is currently a required property. That is a breaking change for any client that requires the key.

1. Merge and deploy clerk_go#21398 first (behavior removal).
2. Ship the tolerant consumers - dashboard and backoffice - before the backend stops sending the field.
3. Then merge the clerk_go API-surface PR.
4. The SDK PRs are independent and can land on their own release cadence; the DAPI OpenAPI override for the SDK-sourced schema stays until clerk_go depends on a clerk-sdk-go release without the field.

Still outstanding after this

The ENHANCED_DELIVERABILITY_POSTMARK_SERVER_TOKEN secrets remain in four SOPS-encrypted files (production and staging, api_common and pubsubworker). They need someone with sops keys. The Postmark account itself also still needs decommissioning.

Changes in this repo

Removes the retired enhancedEmailDeliverability setting from @clerk/backend: the InstanceApi.updateInstance param, the InstanceSettings resource, and its JSON type. The param has been a server-side no-op since the delivery path was retired, and the Backend API now ignores it entirely.

Breaking change to a public type, so a major changeset is included. Flagged as draft for the SDK owners to slot into their release cadence - no rush, the server tolerates clients that still send the field.

Companion PRs

• backoffice: https://github.com/clerk/backoffice/pull/724
• clerk-sdk-go: Remove the enhanced_email_deliverability API surface clerk-sdk-go#599
• clerk_go: https://github.com/clerk/clerk_go/pull/21398
• dashboard: https://github.com/clerk/dashboard/pull/10112
• terraform-app-infra: https://github.com/clerk/terraform-app-infra/pull/3193
• clerk_go:api-surface: https://github.com/clerk/clerk_go/pull/21613

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Project	Deployment	Actions	Updated (UTC)
clerk-js-sandbox	Ready	Preview	Aug 28, 2026 2:55pm
swingset	Ready	Preview	Aug 28, 2026 2:55pm

[changeset-bot]

🦋 Changeset detected

Latest commit: 3d4c750

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 10 packages

Name	Type
@clerk/backend	Major
@clerk/astro	Patch
@clerk/express	Patch
@clerk/fastify	Patch
@clerk/hono	Patch
@clerk/nextjs	Patch
@clerk/nuxt	Patch
@clerk/react-router	Patch
@clerk/tanstack-react-start	Patch
@clerk/testing	Patch

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commit: 3d4c750