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/configs/regen.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+# regen.sh — regenerate per-playlist sldl configs from _template.conf.
+#
+# Source of truth is $ALEMBIC_CONFIG_DIR/pipeline/playlists.json, written by
+# the alembic app's playlist_service whenever a playlist is added/edited/
+# removed via the UI. This script never edits that source of truth — it only
+# renders it. Run automatically by the app before each playlist sync, or
+# manually: regen.sh
+#
+# playlists.json shape: a JSON array of objects, each with at least
+# {"name": "...", "spotify_url": "...", "active": true, "no_m3u": false}.
+# Inactive playlists still get a .conf rendered (so a manual run is always
+# possible) but are skipped by the scheduler.
+
+set -euo pipefail
+
+ALEMBIC_CONFIG_DIR="${ALEMBIC_CONFIG_DIR:-/config}"
+MUSIC_DATA_DIR="${MUSIC_DATA_DIR:-/data/music}"
+PIPELINE_DIR="${PIPELINE_DIR:-/app/pipeline}"
+
+CONFIG_OUT_DIR="$ALEMBIC_CONFIG_DIR/pipeline"
+PLAYLISTS_JSON="$CONFIG_OUT_DIR/playlists.json"
+TEMPLATE="$PIPELINE_DIR/configs/_template.conf"
+SLDL_DROPBOX_ROOT="$MUSIC_DATA_DIR/sldl-dropbox"
+
+if [[ ! -f "$PLAYLISTS_JSON" ]]; then
+  echo "ERROR: $PLAYLISTS_JSON not found — nothing to regenerate." >&2
+  echo "       This file is written by the alembic app; run its playlist" >&2
+  echo "       import/seed step first if this is a fresh setup." >&2
+  exit 1
+fi
+
+mkdir -p "$CONFIG_OUT_DIR"
+
+# One "name<TAB>spotify_url" line per playlist, active or not (see header).
+python3 -c '
+import json, sys
+with open(sys.argv[1]) as f:
+    playlists = json.load(f)
+for p in playlists:
+    print(f"{p[\"name\"]}\t{p[\"spotify_url\"]}")
+' "$PLAYLISTS_JSON" | while IFS=$'\t' read -r name url; do
+  [[ -z "$name" ]] && continue
+  sed -e "s|PLAYLIST_NAME|${name}|g" \
+      -e "s|SPOTIFY_URL|${url}|g" \
+      -e "s|SLDL_DROPBOX_ROOT|${SLDL_DROPBOX_ROOT}|g" \
+      "$TEMPLATE" > "$CONFIG_OUT_DIR/${name}.conf"
+  echo "Generated $CONFIG_OUT_DIR/${name}.conf"
+done