Use gosu for reliable UID:GID switching

Fix PUID/PGID collision issues by using gosu to run services with exact UID:GID. Changes include:

- Added redis-start.sh and updated app-start.sh to load /etc/environment, determine PUID/PGID, and invoke gosu "$PUID:$PGID" to start Redis and the Next.js app (with verification and fallbacks).
- Updated entrypoint.sh to persist PUID/PGID into /etc/environment, document the gosu approach, and adjust startup messaging.
- Updated supervisord.conf to run the new startup wrappers as root (so they can use gosu) instead of running processes directly as specific users.
- Dockerfile updated to install gosu and copy the redis-start.sh wrapper.
- Documentation updated (deployment/unified.md) describing the PUID collision bug, the root cause, and the gosu-based fix.

This resolves cases where PUID collides with existing system users (e.g., nobody) which previously caused processes to run with the wrong GID and produce EACCES errors.

docker/unified/supervisord.conf

@@ -20,8 +20,8 @@ stdout_events_enabled=true
 stderr_events_enabled=true
 
 [program:redis]
-command=/usr/bin/redis-server --appendonly yes --dir /var/lib/redis --bind 127.0.0.1 --port 6379
-user=redis
+command=/app/redis-start.sh
+user=root
 autostart=true
 autorestart=true
 priority=20
@@ -35,7 +35,7 @@ stderr_events_enabled=true
 [program:app]
 command=/app/app-start.sh
 directory=/app
-user=node
+user=root
 autostart=true
 autorestart=true
 priority=30