Per-camera capture interval; ambient recorder; fix 4-seasons one-frame bug

Per-camera capture interval:
  CAPTURE_INTERVAL_<cam> in sky-cam.conf overrides the global value for
  a specific camera.  capture.sh and 4-seasons.sh both respect it.
  Example: south at 30s saves storage; north at 5s gives smoother motion.

4-seasons.sh one-frame bug fix:
  Same root cause as the sunrise scripts — concat list had no 'duration'
  entries, collapsing all JPEG frames to timestamp 0.  Fixed by writing
  'duration INTERVAL' per frame.  Added fps=25 to the speed-adjust step
  so the VFR source resamples to a standard frame rate.

ambient-record.sh (new):
  Continuous AAC recorder for natural sounds (birds, rain, wind).
  Saves AMBIENT_CHUNK_SECS chunks to AMBIENT_DIR/<cam>/YYYY-MM-DD/HH-MM-SS.m4a.
  Reconnects automatically on stream failure.  Rolling retention via
  AMBIENT_RETENTION_DAYS.  Enabled by setting AMBIENT_ENABLED=true and
  listing cameras in AMBIENT_CAMS in sky-cam.conf, then re-running install.sh.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

ambient-record.sh

@@ -0,0 +1,79 @@
+#!/bin/bash
+# ambient-record.sh <cam-name> — continuous ambient audio recorder.
+#
+# Records RTSP audio in AMBIENT_CHUNK_SECS chunks, saves to:
+#   AMBIENT_DIR/<cam>/YYYY-MM-DD/HH-MM-SS.m4a
+#
+# Files are named by the recording start time so the collection is easy to
+# browse and pick from for white-noise / nature sound playback.
+#
+# Runs as a permanent systemd service (Type=simple) alongside capture.sh.
+# Reconnects automatically if the camera stream drops.
+# Applies rolling retention (AMBIENT_RETENTION_DAYS).
+#
+# Camera audio is typically 8 kHz mono (phone quality) — suitable for bird
+# song, rain, wind, and ambient nature sounds; check your camera's web
+# interface for a higher sample rate setting if available.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(dirname "$(realpath "$0")")"
+source "$SCRIPT_DIR/sky-cam.conf"
+source "$SCRIPT_DIR/.env" 2>/dev/null || true
+export TZ="$TIMEZONE"
+
+CAM="${1:-}"
+if [ -z "$CAM" ]; then
+    echo "Usage: $0 <camera-name>"
+    exit 1
+fi
+
+rtsp_var="CAM_RTSP_${CAM}"
+RTSP_URL="${!rtsp_var:-}"
+if [ -z "$RTSP_URL" ]; then
+    echo "ERROR: CAM_RTSP_${CAM} not set in .env"
+    exit 1
+fi
+
+CHUNK="${AMBIENT_CHUNK_SECS:-1800}"
+BITRATE="${AMBIENT_BITRATE:-96k}"
+RETAIN="${AMBIENT_RETENTION_DAYS:-30}"
+SAVE_DIR="${AMBIENT_DIR:-$BASE_DIR/ambient}"
+
+echo "Ambient recorder started: cam=$CAM  chunk=${CHUNK}s  bitrate=$BITRATE  retain=${RETAIN}d"
+echo "Saving to: $SAVE_DIR/$CAM/"
+
+while true; do
+    today=$(date +%Y-%m-%d)
+    dir="$SAVE_DIR/$CAM/$today"
+    mkdir -p "$dir"
+
+    timestamp=$(date +%H-%M-%S)
+    outfile="$dir/${timestamp}.m4a"
+
+    echo "[$(date '+%Y-%m-%d %H:%M:%S')] Recording → $outfile  (${CHUNK}s)"
+
+    ffmpeg -loglevel warning \
+        -rtsp_transport tcp \
+        -i "$RTSP_URL" \
+        -t "$CHUNK" \
+        -vn \
+        -c:a aac -b:a "$BITRATE" \
+        -y "$outfile" || true
+
+    if [ ! -s "$outfile" ]; then
+        echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$CAM] recording failed or empty — retrying in 30s"
+        rm -f "$outfile"
+        sleep 30
+        continue
+    fi
+
+    actual=$(ffprobe -v error -show_entries format=duration -of csv=p=0 "$outfile" 2>/dev/null || echo "?")
+    echo "[$(date '+%Y-%m-%d %H:%M:%S')] Saved: $(basename "$outfile")  (${actual}s)"
+
+    # Rolling retention
+    if [ "${RETAIN}" -gt 0 ]; then
+        deleted=$(find "$SAVE_DIR/$CAM" -name "*.m4a" -mtime +"$RETAIN" -print -delete 2>/dev/null | wc -l)
+        [ "$deleted" -gt 0 ] && echo "Retention: deleted $deleted file(s) older than ${RETAIN}d"
+    fi
+done