Trigger generation of the AI "after treatment" glow-up for a completed skin
analysis — the source face image with detected skin concerns (acne, pigmentation,
texture, etc.) removed while preserving identity, pose and natural skin tone.

Generation runs in the background (~25–60s) and is already auto-triggered at scan
start for mobile-app scans (it runs in parallel with the analysis), so this endpoint
is mainly an on-demand / regenerate trigger. It returns immediately:

• 202 with status: processing when generation has been queued.
• 200 with status: completed and the image URL if a glow-up already exists.

Poll GET /skin/analysis/{batch_id}/glow-up for the result. If no image_id
is specified, the front-facing image from the batch is used.