Add allsky_skyquality: calibrated mag/arcsec² Sky Quality Meter

[benhartwich]

allsky_skyquality — calibrated mag/arcsec² Sky Quality Meter

Adds a new module allsky_skyquality.

What it does

Reports night sky brightness in mag/arcsec² and writes a rolling skyquality.json
for time-series charting — no database or external sensor required.

How it differs from the bundled allsky_sqm

allsky_sqm reports a raw ROI mean ADU. Because Allsky auto-exposes and
auto-gains at night, raw ADU is not comparable between frames (a darker sky just
gets a longer exposure and lands at a similar ADU). This module normalises first:

signal = mean_ADU / exposure_s / gain_factor      gain_factor = 10^(gain / gain_scale)
mag    = offset − 2.5 · log10(signal)

so the reading tracks true sky brightness. offset is a one-time per-camera
calibration constant. It also records three sensor-free extras: template-matched
star count, a cloud index (star-deficit grid) and a candidate aurora index
(green excess on the north horizon).

Notes

• Night event only; fast (single grayscale pass + optional template match).
• Uses only cv2 + numpy (already shipped with Allsky) — no extra requirements.
• Full README, calibration guide and a ready-made charting dashboard:
  https://github.com/benhartwich/allsky-skyquality
• Approach inspired by indi-allsky.

Tested live on a ZWO 4K fisheye camera (astronomy.garden): monotonic night curve,
darkest reading ≈ 20.8 mag/arcsec² after calibration.