feat(seer): Add structured Night Shift errors (#123065)

Night Shift now persists a machine-readable `error_type` alongside
diagnostic error text. The workflows API exposes that value as
`errorType`, allowing the history UI to present errors such as
`no_quota` and `no_seer_access` as skipped without treating backend
prose as an API contract.

Historical records that only contain `error_message` are classified in
the serializer for compatibility. New run and shard errors write stable
codes at their source, while unknown diagnostics remain failed with
`errorType: "unknown"`.

This is the backend prerequisite for the frontend cleanup in #122675 and
should deploy first.