Audio capture records exactly SUNRISE_TARGET_SECS centred on sunrise

Previously sunrise-audio-capture.sh recorded the entire capture window
(70+ minutes) and daily_sunrise_video.sh sought to the right position with
a complex offset calculation.

Now: record floor(TARGET/2) seconds before sunrise and ceil(TARGET/2) after —
odd second goes to post-sunrise.  The file is exactly SUNRISE_TARGET_SECS long
and already centred, so no offset is needed when mixing.

AUDIO_PRE_BUFFER_MIN removed — it was part of the old whole-window scheme
and is no longer meaningful.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

daily_sunrise_video.sh

@@ -171,10 +171,9 @@ audio_offset="0"
 cam_audio="$image_dir/sunrise-audio.m4a"
 if [ "${AUDIO_ENABLED:-false}" = "true" ]; then
     if [ -f "$cam_audio" ]; then
-        buffer_sec=$(( ${AUDIO_PRE_BUFFER_MIN:-2} * 60 ))
-        audio_offset=$(echo "scale=1; $SUNRISE_PRE_MIN * 60 + $buffer_sec - $SUNRISE_TARGET_SECS / 2" | bc)
+        # Already exactly SUNRISE_TARGET_SECS, centred on sunrise — no offset needed
         audio_src="$cam_audio"
-        echo "Audio: camera recording  offset=${audio_offset}s"
+        echo "Audio: camera recording (centred on sunrise)"
     else
         library_dir="$SCRIPT_DIR/sunrise-sounds"
         if [ -d "$library_dir" ]; then