Stage 0: migrate pipeline scripts from /opt/sldl, scaffold repo

Moves all ~30 pipeline scripts, configs, and the vendored sldl binary into
this repo (source /opt/sldl left untouched). Removes all docker exec/docker
compose dependencies now that beets and sldl run in-process/as a subprocess
of this container instead of via soulbeet/on-demand sldl containers.
Replaces hardcoded host paths, Navidrome credentials, and Spotify credential
sourcing with env-var-driven paths and shared credential loaders. Adds
Dockerfile, entrypoint.sh, requirements.txt, docker-compose.snippet.yml, and
the initial app DB schema.

pipeline/lib/gen-vgm-playlist.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# gen-vgm-playlist.sh — regenerate vgm.m3u8 from all tracks tagged with
+# genre "Game Soundtrack" in the beets library.
+#
+# Navidrome picks this up as a regular playlist. Safe to run manually anytime.
+
+set -u
+
+PLAYLIST_DIR=${MUSIC_DATA_DIR:-/data/music}/playlists
+OUT="${PLAYLIST_DIR}/vgm.m3u8"
+LOG_DIR=${ALEMBIC_CONFIG_DIR:-/config}/logs
+LOG="${LOG_DIR}/vgm-$(date +%Y%m%d).log"
+
+mkdir -p "$LOG_DIR" "$PLAYLIST_DIR"
+
+log() { echo "[$(date -Iseconds)] $*" | tee -a "$LOG"; }
+
+log "=== Regenerating vgm.m3u8 ==="
+
+SORTED=$(beet ls -f '$path' 'genres:Game' 2>>"$LOG" | sort -u)
+COUNT=$(echo -n "$SORTED" | grep -c '^' || true)
+
+if [[ "$COUNT" -gt 0 ]]; then
+  {
+    echo "#EXTM3U"
+    echo "$SORTED"
+  } > "$OUT"
+  log "vgm.m3u8 updated with $COUNT tracks"
+else
+  log "WARNING: no tracks found — vgm.m3u8 not updated"
+fi
+
+log "=== Finished ==="