Restructure audio into AUDIO_DIR with per-camera retention

New AUDIO_DIR variable (like BASE_DIR/MOVIES_DIR) is the root for all
audio recordings.  Set in .env to put audio on a separate drive.

Directory layout:
  AUDIO_DIR/<cam>/YYYY-MM-DD/HH-MM-SS.m4a        — ambient recordings
  AUDIO_DIR/sunrise/<cam>/YYYY-MM-DD/sunrise-audio.m4a — sunrise clips

sky-cam.conf:
- Add AUDIO_DIR="${AUDIO_DIR:-$BASE_DIR/audio}" in storage section
- Remove AMBIENT_DIR (replaced by AUDIO_DIR)
- Add SUNRISE_AUDIO_RETENTION_DAYS=7 (sunrise clips kept separately from ambient)
- Add per-camera ambient retention examples:
    AMBIENT_RETENTION_DAYS_south=60  (keep bird recordings longer)

ambient-record.sh:
- Use AUDIO_DIR instead of AMBIENT_DIR
- Per-camera retention: AMBIENT_RETENTION_DAYS_<cam> overrides global default

sunrise-audio-capture.sh:
- Save clips to AUDIO_DIR/sunrise/<cam>/<date>/sunrise-audio.m4a
- Rolling retention of sunrise clips (SUNRISE_AUDIO_RETENTION_DAYS)

daily_sunrise_video.sh:
- Look for cam_audio in new AUDIO_DIR/sunrise path first, fall back to
  old BASE_DIR path so existing recordings keep working

install.sh:
- Pre-create AUDIO_DIR/<cam> and AUDIO_DIR/sunrise/<SUNRISE_CAM> dirs
- Add AUDIO_DIR to .env.example storage section

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

daily_sunrise_video.sh

@@ -242,7 +242,10 @@ if $TEST_MODE; then
         echo "Audio: CAM_RTSP_${CAM_NAME} not set — skipping"
     fi
 else
-    cam_audio="$image_dir/sunrise-audio.m4a"
+    # New path: AUDIO_DIR/sunrise/<cam>/<date>/sunrise-audio.m4a
+    # Fallback to old path (BASE_DIR/<cam>/<date>/sunrise-audio.m4a) for migration
+    cam_audio="${AUDIO_DIR:-$BASE_DIR/audio}/sunrise/$CAM_NAME/$current_date/sunrise-audio.m4a"
+    [ ! -f "$cam_audio" ] && cam_audio="$image_dir/sunrise-audio.m4a"
     if [ "${AUDIO_ENABLED:-false}" = "true" ]; then
         if [ -f "$cam_audio" ]; then
             audio_src="$cam_audio"