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/djmix-albums.txt.example

@@ -0,0 +1,35 @@
+# djmix-albums.txt — albums to include in the djmix.m3u8 exclusion playlist
+#
+# Each non-comment line is a beet query for one album. Lines without a pipe (|)
+# are passed as a single beet query argument. Lines containing | are split on |
+# and each part is passed as a separate beet query term (ANDed together).
+#
+# Use | to combine albumartist and album constraints, e.g.:
+#   albumartist::Porter Robinson | album:Secret Sky 2021
+#
+# Single-field lines with spaces work fine as one arg — beet treats the whole
+# string after the colon as the value. Do NOT word-split commas-in-names.
+#
+# To add an album, find the exact name first:
+#   docker exec soulbeet beet ls -f '$albumartist|$album' 'artist:Name' | sort -u
+# Then add a line and run: sudo /opt/sldl/lib/gen-djmix-playlist.sh
+
+# ---- Fred again.. — Live DJ sets ----
+album:Apple Music Live: Fred again..
+album:Live from Lyon, France, Oct 24, 2025
+album:Alexandra Palace, London, Feb 27, 2026
+album:Live from Toronto, Canada, Nov 14, 2025
+
+# ---- Porter Robinson — Live performances / DJ sets ----
+albumartist::Porter Robinson | album:Secret Sky 2021
+albumartist::Porter Robinson | album:Worlds Live at Second Sky 2019
+albumartist::Porter Robinson | album:Shelter Live
+
+# ---- Virtual Self b2b G Jones ----
+albumartist::Virtual Self | album:Second Sky 2022
+
+# ---- Air2Earth ----
+albumartist::^Air2Earth$ | album:Splash House
+
+# ---- G Jones — add more DJ mix albums below ----
+# Find album names with: docker exec soulbeet beet ls -f '$albumartist|$album' 'artist:"G Jones"' | sort -u