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/artist-canonical.list.example

@@ -0,0 +1,46 @@
+# /opt/sldl/configs/artist-canonical.list
+#
+# Canonical artist-name casing. One name per line; '#' starts a comment.
+# Names here are written exactly as Spotify (or the artist) stylizes them.
+#
+# normalize-artist-casing.py looks up every distinct $albumartist value in the
+# beets library; for any value that matches a name here case-insensitively but
+# differs in casing, it runs `beet modify` to rewrite the tag to the canonical
+# form. beets then moves the album folder to the canonical path.
+#
+# Purpose: prevents Linux → Windows Syncthing collisions like `Mall Grab/` vs
+# `MALL GRAB/` (same path on Windows, different on Linux).
+#
+# To add an artist:
+#   1. Look them up on Spotify, copy their displayed name exactly.
+#   2. Append a new line below.
+#   3. Run /opt/sldl/lib/normalize-artist-casing.py to preview, then --apply.
+#
+# Characters illegal on Windows (`<>:"\?*|`) can appear here — beets's `replace:`
+# rule substitutes them with `_` when generating the on-disk path, so the tag
+# stays Spotify-correct while the folder stays Windows-safe (e.g. `il:lo` tag
+# maps to `il_lo/` folder).
+
+Bicep
+bôa
+Charli xcx
+Charlotte de Witte
+Crush3d
+deadmau5
+DJ HEARTSTRING
+DjRUM
+Doechii
+Eprom
+FM Forest
+il:lo
+KETTAMA
+Mall Grab
+nanobii
+Pasocom Music Club
+Porter Robinson
+Ross from Friends
+Shades
+Svetec
+UMEK
+yuragi
+YOTTO