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/convert-m4a.sh

@@ -0,0 +1,57 @@
+#!/bin/bash
+# Convert m4a files in-place: AAC -> MP3 (V0 VBR), ALAC -> FLAC (lossless).
+# Preserves metadata + embedded cover art. Deletes original on success.
+set -u
+
+LOG=${ALEMBIC_CONFIG_DIR:-/config}/logs/convert-m4a-$(date +%Y%m%d-%H%M%S).log
+mkdir -p "$(dirname "$LOG")"
+
+log() { echo "[$(date -Iseconds)] $*" | tee -a "$LOG"; }
+
+OK_AAC=0
+OK_ALAC=0
+FAIL=0
+
+while IFS= read -r -d '' f; do
+  codec=$(ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of csv=p=0 "$f" 2>/dev/null | tr -d ',')
+  base="${f%.m4a}"
+  case "$codec" in
+    aac)
+      out="${base}.mp3"
+      if [[ -e "$out" ]]; then log "SKIP-EXISTS: $out"; FAIL=$((FAIL+1)); continue; fi
+      if ffmpeg -nostdin -hide_banner -loglevel error -i "$f" \
+           -map 0 -c:v copy -c:a libmp3lame -q:a 0 -id3v2_version 3 \
+           "$out" 2>>"$LOG"; then
+        rm -f "$f"
+        log "AAC→MP3:    $out"
+        OK_AAC=$((OK_AAC+1))
+      else
+        log "FAIL-AAC:   $f"
+        rm -f "$out" 2>/dev/null
+        FAIL=$((FAIL+1))
+      fi
+      ;;
+    alac)
+      out="${base}.flac"
+      if [[ -e "$out" ]]; then log "SKIP-EXISTS: $out"; FAIL=$((FAIL+1)); continue; fi
+      if ffmpeg -nostdin -hide_banner -loglevel error -i "$f" \
+           -map 0 -c:v copy -c:a flac -compression_level 8 \
+           "$out" 2>>"$LOG"; then
+        rm -f "$f"
+        log "ALAC→FLAC:  $out"
+        OK_ALAC=$((OK_ALAC+1))
+      else
+        log "FAIL-ALAC:  $f"
+        rm -f "$out" 2>/dev/null
+        FAIL=$((FAIL+1))
+      fi
+      ;;
+    *)
+      log "SKIP-OTHER ($codec): $f"
+      ;;
+  esac
+done < <(find ${MUSIC_DATA_DIR:-/data/music}/Library -type f -iname "*.m4a" -print0 2>/dev/null)
+
+log ""
+log "=== AAC→MP3:  $OK_AAC | ALAC→FLAC: $OK_ALAC | Failed: $FAIL ==="
+log "Log: $LOG"