feat(preview-deployments): Custom Entrypoint support

[disc]

Summary

• Add a Custom Entrypoint setting to an application's Preview Deployment configuration, so auto-generated preview domains can override the default web/websecure Traefik entrypoints — matching the Custom Entrypoint control already available on regular application domains.
• New nullable applications.previewCustomEntrypoint column (+ migration 0173). createPreviewDeployment passes it through to createDomain, and the existing manageDomain call writes it into the Traefik dynamic config automatically.
• Preview Settings gets a Custom Entrypoint toggle + Entrypoint Name input; toggling off stores null and reverts to the default. Only affects previews created after the setting is saved.

Validation

• Tested locally end-to-end (GitHub App + tunnel): set a custom entrypoint, opened a PR, and confirmed the generated preview domain's Traefik router uses the configured entrypoint; toggling off reverts to web/websecure.

preview-deployments-custom-entrypoint.mp4

Issues related

Closes #4796