Connect jellyfin to npm-network for internal reverse-proxy access

Enables an nginx-proxy-manager proxy host (jellyfin:8096) so it can be
reached by hostname on the LAN instead of IP:port. No public
VIRTUAL_HOST/LETSENCRYPT_HOST since this stays internal-only for now.

Claude-Session: https://claude.ai/code/session_01HZQK6jHmdTpFjFZM8FUnqA

jellyfin/docker-compose.yaml

@@ -21,6 +21,9 @@ services:
       - /srv/jellyfin/config:/config
       - /srv/jellyfin/cache:/cache
       - nas_media:/data
+    networks:
+      - default
+      - npm-network
     deploy:
       resources:
         reservations:
@@ -28,3 +31,8 @@ services:
             - driver: nvidia
               count: all
               capabilities: [gpu]
+
+networks:
+  default:
+  npm-network:
+    external: true