Harden gen-djmix, gen-vgm, strip-mb-tags, notify-telegram under set -euo pipefail (R1)

Adds strict mode to four maintenance scripts, with guards so expected non-zero
exits (empty beet queries, a missing config, a curl timeout) log/skip instead
of aborting: gen-djmix bails cleanly if djmix-albums.txt is absent and tolerates
a bad single query; gen-vgm tolerates an empty query; strip-mb-tags quotes its
path arg and best-efforts the trailing beet update; notify-telegram guards curl
so a network error still reaches its explicit "send failed" branch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipeline/lib/gen-vgm-playlist.sh

@@ -4,7 +4,7 @@
 #
 # Navidrome picks this up as a regular playlist. Safe to run manually anytime.
 
-set -u
+set -euo pipefail
 
 PLAYLIST_DIR=${MUSIC_DATA_DIR:-/data/music}/playlists
 OUT="${PLAYLIST_DIR}/vgm.m3u8"
@@ -17,7 +17,9 @@ log() { echo "[$(date -Iseconds)] $*" | tee -a "$LOG"; }
 
 log "=== Regenerating vgm.m3u8 ==="
 
-SORTED=$(beet ls -f '$path' 'genres:Game' 2>>"$LOG" | sort -u)
+# `|| true`: an empty or failed query just means no tracks, handled below as a
+# warning. This is a cosmetic playlist regen, not worth aborting the job over.
+SORTED=$(beet ls -f '$path' 'genres:Game' 2>>"$LOG" | sort -u || true)
 COUNT=$(echo -n "$SORTED" | grep -c '^' || true)
 
 if [[ "$COUNT" -gt 0 ]]; then