chore: improve production deployment flexibility

- Switch frontend from nginx to caddy for consistency with Coolify
- Make VITE_API_URL optional, auto-derive from window.location.origin/api
- Remove hardcoded port mappings, let Coolify/Traefik handle routing
- Simplifies deployment configuration

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -29,8 +29,6 @@ services:
       ZULIP_EMAIL: ${ZULIP_EMAIL:-}
       ZULIP_API_KEY: ${ZULIP_API_KEY:-}
       ZULIP_STREAM: ${ZULIP_STREAM:-general}
-    ports:
-      - "8000:8000"
     depends_on:
       db:
         condition: service_healthy
@@ -42,8 +40,6 @@ services:
       dockerfile: Dockerfile.prod
       args:
         VITE_API_URL: ${VITE_API_URL:-}
-    ports:
-      - "8080:8080"
     depends_on:
       - backend
     restart: unless-stopped