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

notify.sh

@@ -52,7 +52,7 @@ fi
 
 # ── Mattermost text post ──────────────────────────────────────────────────────
 # mattermost_url and access_token come from sky-cam.conf (sourced above).
-if [ "$MM_NOTIFY_ENABLED" = "true" ] && [ -n "$MM_NOTIFY_CHANNEL_ID" ]; then
+if [ "$MM_NOTIFY_ENABLED" = "true" ] && [ -n "$MM_NOTIFY_CHANNEL_ID" ] && [ -n "${mattermost_url:-}" ] && [ -n "${access_token:-}" ]; then
     curl -s \
         -H "Authorization: Bearer $access_token" \
         -H "Content-Type: application/json" \