feat(model-manager): run container as host UID/GID

Downloads previously landed in models/ owned by root because the
container ran as root. Add `user: "${PUID:-1000}:${PGID:-1000}"` to the
model-manager service and PUID/PGID to .env.example so downloaded models
are owned by the host user. Defaults to 1000:1000.

Note: existing root-owned files under models/ and sparkyui-data/ must be
chowned once (e.g. via a one-off root container) when upgrading.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -138,6 +138,10 @@ services:
     image: sparkyui-model-manager:latest
     container_name: model-manager
 
+    # Run as the host user so downloaded models are owned by you, not root.
+    # Defaults to 1000:1000; override via PUID/PGID in .env if needed.
+    user: "${PUID:-1000}:${PGID:-1000}"
+
     ports:
       - "${MODEL_MANAGER_PORT:-8189}:8189"