sunrise.py: replace suntime with skyfield for accurate sunrise calculation

The suntime library uses a simplified approximation that degrades at high
latitudes as sunrise approaches very early hours near the summer solstice.
The computed sunrise drifted progressively earlier than the actual event,
shrinking the capture window until today it ended before the sun rose.

Switch to skyfield's sunrise_sunset almanac (DE421 ephemeris), which is
accurate to within a second at any latitude — the same library already
used by moon_phase.py for sun events. The search window is anchored to
local midnight→midnight UTC so the correct calendar-day sunrise is
always returned regardless of timezone offset.

Also promote skyfield to the core pip install in bootstrap.sh (dropping
suntime, which is no longer needed) and tidy the stale "suntime" hints
in 4-seasons.sh and montage-mvt.sh error messages.

https://claude.ai/code/session_01S4oUbU9xNj91cbV5bj5NRn

bootstrap.sh

@@ -22,8 +22,8 @@ echo ""
 echo "Done. Next steps:"
 echo "  1. Install system packages (if not already present):"
 echo "       sudo apt install ffmpeg bc fonts-dejavu"
-echo "       pip3 install suntime pytz requests"
-echo "       pip3 install skyfield Pillow numpy scipy   # moon jobs (moon-track, moon-phase-monthly)"
+echo "       pip3 install pytz requests skyfield"
+echo "       pip3 install Pillow numpy scipy   # moon jobs (moon-track, moon-phase-monthly)"
 echo ""
 echo "  2. Edit $TARGET/sky-cam.conf"
 echo "     — SCRIPT_DIR   full path to the directory you'll run scripts from"