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/prep-audio.sh

@@ -0,0 +1,50 @@
+#!/bin/bash
+# ${PIPELINE_DIR:-/app/pipeline}/lib/prep-audio.sh
+#
+# Shared helper sourced by run-playlist.sh and import-track.sh.
+# Pre-tags newly-arrived audio with ReplayGain (loudgain, track-mode) and
+# Liquidsoap autocue tags (cue_file from Moonbase59/autocue) before beets
+# ingests them, so AzuraCast sees fully-prepped files the moment they land
+# in /music.
+#
+# Usage:  prep_audio <scan_dir> <log_file>
+# Returns: number of files processed (via stdout)
+
+prep_audio() {
+  local scan_dir="$1"
+  local log="$2"
+
+  local files=()
+  while IFS= read -r -d '' f; do
+    files+=("$f")
+  done < <(find "$scan_dir" -type f \
+              \( -iname "*.flac" -o -iname "*.mp3" -o -iname "*.m4a" \
+              -o -iname "*.ogg" -o -iname "*.opus" \) -print0)
+
+  if [[ ${#files[@]} -eq 0 ]]; then
+    echo 0
+    return 0
+  fi
+
+  echo "[prep-audio] loudgain (track-mode RG, clip-safe) on ${#files[@]} file(s)" >>"$log"
+  # -q quiet, -k lower track gain to avoid clipping >-1 dBFS,
+  # -s e write extended ReplayGain2 + R128 tags, -I 4 ID3v2.4 for MP3.
+  # No -a, so each file gets its own track gain (track-mode).
+  if ! loudgain -q -k -s e -I 4 "${files[@]}" >>"$log" 2>&1; then
+    echo "[prep-audio] loudgain returned non-zero (see log)" >>"$log"
+  fi
+
+  echo "[prep-audio] cue_file (autocue liq_* tags) on ${#files[@]} file(s)" >>"$log"
+  # -w write tags into file, -k true-peak clipping prevention.
+  # Skips files that already have liq_cue_file=true (use -f to force).
+  local failed=0
+  for f in "${files[@]}"; do
+    if ! cue_file -w -k "$f" >>"$log" 2>&1; then
+      echo "[prep-audio] cue_file failed on $f" >>"$log"
+      failed=$((failed + 1))
+    fi
+  done
+  [[ $failed -gt 0 ]] && echo "[prep-audio] cue_file errors: $failed" >>"$log"
+
+  echo "${#files[@]}"
+}