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

sky-cam.conf

@@ -287,8 +287,16 @@ MONTAGE_MUSIC_LOOPS_Spring_2=2
 # CAPTURE_STALE_SECS should always be at least 2× CAPTURE_INTERVAL.
 CAPTURE_INTERVAL=10
 
+# Per-camera overrides — uncomment and set to override the global value above.
+# Useful when cameras serve different purposes (e.g. south records ambient audio
+# so fewer frames is fine; north faces a busy scene and benefits from 5s).
+#CAPTURE_INTERVAL_east=10
+#CAPTURE_INTERVAL_north=10
+#CAPTURE_INTERVAL_south=30
+
 # Seconds without a new frame before the watchdog sends a stall notification.
 # A recovery notification fires automatically when capture resumes.
+# Must always be at least 2× the effective CAPTURE_INTERVAL for that camera.
 CAPTURE_STALE_SECS=30
 
 # ── Sunrise audio capture ──────────────────────────────────────────────────────
@@ -302,6 +310,19 @@ CAPTURE_AUDIO_BITRATE=96k     # bitrate for the sunrise audio recording
 # Audio is recorded for exactly SUNRISE_TARGET_SECS, centred on actual sunrise:
 # floor(TARGET/2) seconds before, ceil(TARGET/2) after (odd second → post-sunrise).
 
+# ── Ambient audio library ──────────────────────────────────────────────────────
+# Continuously records natural sounds (birds, rain, wind) from AMBIENT_CAMS to
+# build a reusable audio library.  Runs as a permanent systemd service alongside
+# capture.sh.  Camera audio is 8 kHz mono (phone quality) — enough for nature
+# sounds and white-noise style playback.
+#
+AMBIENT_ENABLED=false
+AMBIENT_CAMS=(south)                    # cameras to record from
+AMBIENT_DIR="${BASE_DIR}/ambient"       # root storage directory
+AMBIENT_CHUNK_SECS=1800                 # seconds per file (default 30 min)
+AMBIENT_BITRATE=96k                     # AAC bitrate; 64–96k is plenty at 8 kHz
+AMBIENT_RETENTION_DAYS=30              # delete files older than this; 0 = keep forever
+
 # ── Mattermost — daily sunrise upload ─────────────────────────────────────────
 # mattermost_url, access_token, channel_id go in .env (see bottom of this file).