feat: play real music from local library in Memories slideshow

Scans MUSIC_DIR (mounted at /music in Docker) with music-metadata to
read embedded BPM/genre tags and classify tracks as upbeat/cinematic/
dreamy. Falls back to untagged general pool when mood bucket is thin.
Slideshow plays real audio via <audio> element with range-request
streaming; Web Audio synth is kept as fallback when MUSIC_DIR is unset.
Shows now-playing title/artist in the slideshow bottom bar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -11,6 +11,7 @@ services:
       PORT: "3000"
       DB_PATH: /data/library.db
       POSTER_DIR: /data/posters
+      MUSIC_DIR: /music
       NODE_ENV: production
     volumes:
       - type: volume
@@ -19,6 +20,10 @@ services:
       - type: bind
         source: /media/Plex/Gallery
         target: /photos
+      - type: bind
+        source: /DATA/Media/Music
+        target: /music
+        read_only: true
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:3000/api/health"]
       interval: 30s