Poll for the AI "after treatment" glow-up of a scan batch. Returns the current
status and, once ready, the generated image URL.

Self-healing: if no glow-up exists yet for the batch (e.g. an older scan from
before the feature shipped, or one that was never triggered), polling this
endpoint kicks off generation on demand and returns status: processing.
Keep polling until status becomes completed (with glow_up_url) or
failed. Generation is deduplicated, so repeated polls will not start
multiple jobs.