Fix six bugs found in final audit

montage-mvt.sh, year-end-join.sh: CAM_NAME fallback referenced undefined
\$CAM_NAME — crashes under set -euo pipefail on manual runs without args.
Fixed to fall back to \$SUNRISE_CAM (same fix applied earlier to other scripts).

notify.sh: Mattermost notification guard only checked MM_NOTIFY_CHANNEL_ID,
not mattermost_url or access_token. If those are unset, curl would send a
malformed request silently. Added guards for all three required values.

install.sh: seasons/fullday service units had literal \n in After=/Wants=
lines because bash does not interpret \n in heredoc variable expansions.
Changed to \$'...' syntax so actual newlines are written, making valid
systemd unit files. Also: audio capture timer now uses \$SCHEDULE_SUNRISE
instead of hardcoded 03:00, keeping it in sync if the user changes the
schedule.

sunrise-audio-capture.sh: record_start_sec could theoretically go negative
(sunrise very early + large SUNRISE_TARGET_SECS). Added floor-at-zero guard.

daily_sunrise_video.sh: added comment explaining why audio_offset is always 0.

README.md: fix CAM_RTSP_sunrise example to CAM_RTSP_east.

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

daily_sunrise_video.sh

@@ -168,7 +168,7 @@ fade_out=$(echo "scale=1; $SUNRISE_TARGET_SECS - 0.5" | bc)
 
 # ── Pick audio source ─────────────────────────────────────────────────────────
 audio_src=""
-audio_offset="0"
+audio_offset="0"  # cam audio is pre-centred on sunrise; library files are trimmed to fit
 
 cam_audio="$image_dir/sunrise-audio.m4a"
 if [ "${AUDIO_ENABLED:-false}" = "true" ]; then