Expand obs window to full dark+moon period; label with illumination%

moon_phase.py: add sun_altaz() to compute sun altitude for dark-sky check

moon_phase_monthly.py:
- Replace fixed 22:00-23:00 window with dynamic dark+moon intervals:
  sample every 10 min across ±24h of the phase moment, keep blocks where
  sun < DARK_SKY_SUN_ALT_DEG and moon > MIN_ALTITUDE — covers the full
  night the moon is actually visible in a dark sky regardless of month
- First east frame with quality >= MIN_QUALITY in that window wins
- Illumination% computed at the detection time and shown in the image
  caption and Mattermost message
- Remove OBS_TIME_H/M; add DARK_SKY_SUN_ALT_DEG config var

sky-cam.conf: replace MOON_OBS_TIME_LOCAL with MOON_DARK_SKY_SUN_ALT_DEG=-6
and explain the sun-altitude threshold options

https://claude.ai/code/session_01HkTxpNSTWtViZzxbrbKytR

sky-cam.conf

@@ -491,19 +491,21 @@ MOON_TRACK_FPS=12                  # output mp4 framerate
 MOON_TRACK_CRF=24                  # output mp4 CRF
 MOON_TRACK_RETENTION_DAYS=90       # delete tracker mp4s older than this; 0 = forever
 
-# Observation time ────────────────────────────────────────────────────────────
-# Local time that defines the observation window: MOON_OBS_TIME_LOCAL to
-# MOON_OBS_TIME_LOCAL+1h.  East frames in this window are checked for a clear
-# moon detection (quality >= MOON_MIN_QUALITY).  If any frame qualifies, the
-# NASA Dial-a-Moon image for MOON_OBS_TIME_LOCAL UTC is fetched and posted.
-# If no frame shows a clear moon, the month is skipped entirely.
+# Dark-sky observation window ─────────────────────────────────────────────────
+# The script searches ±24h around the exact phase moment for east frames where:
+#   1. The sun is below MOON_DARK_SKY_SUN_ALT_DEG  (sky is dark)
+#   2. The moon is above MOON_MIN_ALTITUDE_DEG      (moon is visible above trees)
 #
-# 22:00 aligns directly with NASA's hourly renders.  Full moon and third
-# quarter both rise after dark and are visible at this hour.  First quarter
-# is up only until ~midnight from a new-moon start, so it may not be visible
-# at 22:00 depending on the exact date — set MOON_FIRST_QUARTER_ENABLED=false
-# if first-quarter posts are consistently missed.
-MOON_OBS_TIME_LOCAL=22:00
+# Sun altitude thresholds:
+#   -6  civil twilight  — noticeably dark, bright stars visible   (default)
+#   -12 nautical        — horizon barely visible
+#   -18 astronomical    — fully dark, no twilight glow
+#
+# The first east frame in that window with quality >= MOON_MIN_QUALITY is used.
+# Its exact timestamp drives the NASA Dial-a-Moon fetch (rounded to nearest hour)
+# and the parallactic angle rotation.  The caption shows illumination% at that
+# moment.  If no clear frame is found, the month is skipped entirely.
+MOON_DARK_SKY_SUN_ALT_DEG=-6
 
 # Post delay ──────────────────────────────────────────────────────────────────
 # How many days after the exact phase to run the post.  The post delay gives