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-djmix-playlist.sh

@@ -10,7 +10,7 @@
 #
 # Runs daily from cron. Safe to run manually anytime.
 
-set -u
+set -euo pipefail
 
 CONFIG=${ALEMBIC_CONFIG_DIR:-/config}/pipeline/djmix-albums.txt
 PLAYLIST_DIR=${MUSIC_DATA_DIR:-/data/music}/playlists
@@ -24,6 +24,11 @@ log() { echo "[$(date -Iseconds)] $*" | tee -a "$LOG"; }
 
 log "=== Regenerating djmix.m3u8 ==="
 
+if [[ ! -f "$CONFIG" ]]; then
+  log "no djmix-albums.txt at $CONFIG — nothing to generate"
+  exit 0
+fi
+
 TRACKS=$(mktemp)
 
 while IFS= read -r query; do
@@ -40,7 +45,8 @@ while IFS= read -r query; do
     _t="${_t%"${_t##*[![:space:]]}"}"
     [[ -n "$_t" ]] && _args+=("$_t")
   done
-  beet ls -f '$path' "${_args[@]}" 2>>"$LOG" >> "$TRACKS"
+  # `|| true`: a malformed single query shouldn't abort the whole regen.
+  beet ls -f '$path' "${_args[@]}" 2>>"$LOG" >> "$TRACKS" || true
 done < "$CONFIG"
 
 # Deduplicate and sort