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/genres-whitelist.txt.example

@@ -0,0 +1,112 @@
+Acoustic
+Afrobeat
+Alternative
+Alternative Rock
+Ambient
+Anime
+Black Metal
+Bluegrass
+Blues
+Bossa Nova
+Brazilian
+Breakbeat
+Cantopop
+Chicago House
+Chill
+Classical
+Club
+Country
+Dance
+Dancehall
+DJ Mix
+Death Metal
+Deep House
+Detroit Techno
+Disco
+Drum & Bass
+Dub
+Dubstep
+EDM
+Electro
+Electronic
+Emo
+Folk
+Funk
+Future Bass
+Game Soundtrack
+Garage
+Gospel
+Goth
+Grime
+Grindcore
+Grunge
+Hard Rock
+Hardcore
+Hardstyle
+Heavy Metal
+Hip-Hop
+Honky-Tonk
+House
+IDM
+Indian
+Indie
+Indie Pop
+Indie Rock
+Industrial
+J-Dance
+J-Idol
+J-Pop
+J-Rock
+Jazz
+Jungle
+K-Pop
+Latin
+Lo-Fi
+MPB
+Mandopop
+Metal
+Metalcore
+Minimal Techno
+New Age
+OPM
+Opera
+Pagode
+Phonk
+Piano
+Pop
+Pop Film
+Pop Rock
+Post-Dubstep
+Power Pop
+Progressive House
+Psych Rock
+Punk
+Punk Rock
+R&B
+Rap
+Reggae
+Reggaeton
+Rock
+Rock & Roll
+Rockabilly
+Salsa
+Samba
+Sertanejo
+Shoegaze
+Show Tunes
+Singer-Songwriter
+Ska
+Soul
+Soundtrack
+Synth Pop
+Synthwave
+Tango
+Tech House
+Techno
+Trance
+Trap
+Trip-Hop
+Turkish
+UK Garage
+Vaporwave
+World Music