Block deployment when backend port is already in use

Prevent port conflicts by hard-failing the deployment:

- Change port conflict from warning to fatal error
- Provide clear suggested port and diagnostic commands
- Deployment stops immediately with actionable error message
- User must fix port conflict before deploying

Author: devalade

lib/commands/deploy.sh

@@ -36,9 +36,14 @@ deploy_backend() {
     if ! check_remote_port_available "$BACKEND_PORT"; then
         local suggested_port
         suggested_port=$(suggest_available_port "$BACKEND_PORT")
-        warn "Port $BACKEND_PORT is already in use on $SSH_HOST"
-        warn "Consider updating shipnode.conf: BACKEND_PORT=$suggested_port"
-        warn "Or check running apps with: ssh $SSH_USER@$SSH_HOST -p $SSH_PORT 'ss -tln | grep LISTEN'"
+        error "Port $BACKEND_PORT is already in use on $SSH_HOST
+
+Suggested fix:
+  1. Update shipnode.conf: BACKEND_PORT=$suggested_port
+  2. Check what's using the port:
+     ssh $SSH_USER@$SSH_HOST -p $SSH_PORT 'ss -tln | grep :$BACKEND_PORT'
+
+Deployment blocked to prevent port conflict."
     fi
 
     if [ "$ZERO_DOWNTIME" = "true" ]; then