feat: take a photo on non-anonymous donations

Captures a single webcam frame via nokhwa (V4L2 on Linux, AVFoundation
on macOS) and saves it as a JPEG under the configurable photos_dir
whenever a donation completes with a non-"anon" username, both on
manual "Done" and on inactivity auto-approve. Capture runs on its own
thread and is best-effort so a missing/busy camera never blocks the
donation flow.

Author: eupn