Initial commit: playlink music library hardlink deduplicator

Go backend (chi, SQLite) with an embedded React/TypeScript UI that finds
songs present in both a host and a slave music library and replaces the
slave copy with a hardlink to the host copy.

- Path-first matching with format/title/album-folder fallbacks
- Auto-linking of byte-identical files, re-verified with cmp before linking
- Manual review with ffprobe quality comparison, downgrade guard and
  promote-to-host override
- Build mode to populate an empty slave library from the host
- Dry run by default, atomic stage+rename replacement, quarantine with
  retention, undo, path confinement and authenticated UI
- Dockerfile and compose file for Unraid

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,22 @@
+services:
+  playlink:
+    build: .
+    image: playlink:latest
+    container_name: playlink
+    # Run as the user that owns your music (Unraid: nobody:users = 99:100).
+    user: "99:100"
+    ports:
+      - "7480:7480"
+    environment:
+      - TZ=America/Los_Angeles
+      - PLAYLINK_ALLOWED_ROOTS=/data
+      # Set to true when serving over HTTPS (e.g. behind a reverse proxy).
+      - PLAYLINK_SECURE_COOKIES=false
+    volumes:
+      - /mnt/user/appdata/playlink:/config
+      # IMPORTANT: mount ONE folder that contains BOTH libraries. Hardlinks
+      # cannot cross Docker bind mounts, even on the same disk.
+      #   host  (Albums)  -> /data/Music/Albums
+      #   slave (Singles) -> /data/Music/Instances/cdnutter/Singles
+      - /mnt/user/Sapphire/Media/Music:/data/Music
+    restart: unless-stopped