Use real sunset/sunrise for dark window; pick frame closest to exact phase

moon_phase.py: add sun_events_in_range() — returns actual sunset/sunrise
transitions via skyfield almanac, accurate to within a minute

moon_phase_monthly.py:
- _dark_moon_intervals(): replace sun-altitude threshold sampling with
  actual sunset/sunrise times; dark window = sunset + DARK_START_MIN to
  next sunrise; moon-above-horizon check still sampled every 10 min within
  each night; polar fallback if no sun events found
- run_phase(): sort dark-window frames by proximity to exact phase moment
  before scanning; first clear detection = the frame temporally closest to
  the moon being precisely full/at quarter, not just the first in time order
- Replace DARK_SKY_SUN_ALT_DEG with DARK_START_MIN (default 30 min)

sky-cam.conf: replace MOON_DARK_SKY_SUN_ALT_DEG with MOON_DARK_START_MIN=30;
update comments to explain ±24h window, sunset+30 start, closest-frame logic,
and how the 01:30 and 23:55 edge cases are handled

https://claude.ai/code/session_01HkTxpNSTWtViZzxbrbKytR

sky-cam.conf

@@ -492,20 +492,29 @@ MOON_TRACK_CRF=24                  # output mp4 CRF
 MOON_TRACK_RETENTION_DAYS=90       # delete tracker mp4s older than this; 0 = forever
 
 # 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)
+# The script searches ±24h around the exact phase moment using actual computed
+# sunset and sunrise times for the configured location.  The dark window for
+# each night starts at (sunset + MOON_DARK_START_MIN) and ends at sunrise.
+# Within that window, only frames where the moon is above MOON_MIN_ALTITUDE_DEG
+# are considered.
 #
-# 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 east camera faces east, so the moon is visible to it from moonrise through
+# roughly south — typically from dusk through midnight for a full moon.
+# MOON_DARK_START_MIN=30 means the script starts looking 30 min after the sun
+# sets, when the sky is dark enough for a clean moon shot but east can still
+# catch the moon low on the eastern horizon.
 #
-# 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
+# Of all qualifying frames, the one temporally closest to the exact phase moment
+# is used (not the first one).  This picks the frame when the moon was most
+# precisely full / at quarter.  Its timestamp drives the NASA Dial-a-Moon fetch
+# 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
+#
+# If the phase falls early in the day (e.g. 01:30 local), the ±24h window
+# covers the previous evening through the following dusk — both nights where
+# the moon is essentially full.  If the phase falls just before midnight
+# (e.g. 23:55), both the same evening and the next morning are included.
+MOON_DARK_START_MIN=30
 
 # Post delay ──────────────────────────────────────────────────────────────────
 # How many days after the exact phase to run the post.  The post delay gives