Add zero-downtime deployment scripts using Docker Swarm

Add Docker Swarm deployment configuration and scripts to enable zero-downtime deployments.

Changes

• Update docker/docker-compose.yml with Swarm deploy config (order: start-first)
• Add scripts/deploy-swarm.sh for deploying with image pre-pull and health validation
• Add scripts/rollback-swarm.sh for quick rollback

Usage

# One-time setup
docker swarm init

# Deploy
./scripts/deploy-swarm.sh VERSION --env-file=/path/to/.env

# Rollback
./scripts/rollback-swarm.sh