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>

.env.example

@@ -31,3 +31,8 @@ COMFYUIMINI_REF=main
 # Model Manager - StabilityMatrix-style model download/management UI
 # Access at http://<host>:8189
 MODEL_MANAGER_PORT=8189
+
+# Run the Model Manager container as this host user/group so downloaded
+# models are owned by you (not root). Set to your `id -u` / `id -g`.
+PUID=1000
+PGID=1000