## Summary
- Adds a server-side guard that rejects deletion of standard fields
(`isCustom: false`) in the `deleteOneField` path
- The UI already prevents this, but the API had no enforcement, allowing
standard fields to be deleted via direct GraphQL calls
Without this guard, deleting a standard field like `jobTitle` cascades
to drop dependent generated columns (e.g. `searchVector`), leaving the
object in a broken state where all subsequent queries fail with "Data
validation error."
## Test plan
- [x] Call `deleteOneField` with a standard field ID → should return
`FIELD_MUTATION_NOT_ALLOWED` error
- [x] Call `deleteOneField` with a custom field ID → should succeed as
before
- [x] UI deactivation of standard fields still works (deactivate !=
delete)
Thomas Trompette
772b807490