v0.1.31

v0.1.31 — automatic brightness correction

Corrects a dark/bright capture in the pixels instead of re-shooting it:
metered frames are pulled onto a target with a bounded gamma curve on a
background worker, keeping the untouched original for compare/undo and
shipping it in the download zip. RAW is never touched.

Needs python3-pil. New SD-card images include it; an existing appliance
can install it from Setup ("Install brightness support") with no SSH.

Not yet exercised on the Pi — first run is a bring-up.