feat: replace Nginx with Caddy for automatic SSL management

- Add Caddyfile with automatic HTTPS configuration
- Add Dockerfile.caddy for frontend build with Caddy
- Update docker-compose.yml to use Caddy service
- Update DEPLOY.md with simplified deployment instructions

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

deploy/Caddyfile

@@ -0,0 +1,33 @@
+{
+    email your-email@example.com
+}
+
+your-domain.com {
+    # Mobile client (default)
+    handle {
+        root * /srv/mobile
+        try_files {path} /index.html
+        file_server
+    }
+
+    # Screen client
+    handle /screen/* {
+        root * /srv/screen
+        uri strip_prefix /screen
+        try_files {path} /index.html
+        file_server
+    }
+
+    # API proxy
+    handle /api/* {
+        reverse_proxy server:3000
+    }
+
+    # WebSocket proxy
+    handle /socket.io/* {
+        reverse_proxy server:3000
+    }
+
+    # Gzip compression
+    encode gzip
+}