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

moon_phase.py

@@ -179,6 +179,22 @@ def altaz(when: datetime, lat: float | None = None, lon: float | None = None):
     return float(alt.degrees), float(az.degrees)
 
 
+def sun_altaz(when: datetime, lat: float | None = None, lon: float | None = None):
+    """Apparent altitude/azimuth of the sun in degrees as seen from (lat, lon)."""
+    _lazy()
+    from skyfield.api import wgs84
+    if lat is None and lon is None:
+        obs = _observer
+    else:
+        obs = _eph['earth'] + wgs84.latlon(
+            LATITUDE if lat is None else lat,
+            LONGITUDE if lon is None else lon,
+        )
+    t = _t(when)
+    alt, az, _ = obs.at(t).observe(_eph['sun']).apparent().altaz()
+    return float(alt.degrees), float(az.degrees)
+
+
 def parallactic_angle(when: datetime, lat: float | None = None, lon: float | None = None) -> float:
     """Parallactic angle in degrees — rotates a moon image so celestial north is up.