Consolidate config into sky-cam.conf; systemd failure notifications

sky-cam.conf (new)
- Single file replaces mattermost_config.txt reference, notify_config.txt,
  and the hardcoded BASE_DIR/MUSIC_DIR/TIMEZONE scattered across scripts
- Bash-sourceable and Python key=value readable (same format)

All .sh scripts
- source sky-cam.conf at startup; BASE_DIR / MUSIC_DIR from config

sunrise2mm.py
- Reads sky-cam.conf relative to script location; no hardcoded path

notify.sh
- Sources sky-cam.conf; Mattermost credentials come from there directly,
  no separate mattermost_config.txt lookup needed

systemd/sky-cam-notify-failure@.service (new)
- Template unit called by OnFailure= in each service
- Sends notification via notify.sh when any sky-cam job fails
- All three .service units now have OnFailure=sky-cam-notify-failure@%n.service

Removed: notify_config.txt, sky-cam.crontab (superseded)

https://claude.ai/code/session_01C4jbd3waXG3eKZYbGUjLUQ

year-end-join.sh

@@ -13,6 +13,7 @@
 set -euo pipefail
 
 SCRIPT_DIR="$(dirname "$(realpath "$0")")"
+source "$SCRIPT_DIR/sky-cam.conf"
 
 ASTRO_YEAR="${1:-}"
 if [ -z "$ASTRO_YEAR" ]; then
@@ -20,7 +21,7 @@ if [ -z "$ASTRO_YEAR" ]; then
     exit 1
 fi
 
-base_dir="/home/motion/drives/local-2tb"
+base_dir="$BASE_DIR"
 script_name=$(basename "$SCRIPT_DIR" | cut -d'-' -f1)
 year_dir="$base_dir/movies/$script_name/$ASTRO_YEAR"