Add Caddy reverse proxy with automatic HTTPS

- Add Caddyfile: proxies to explorer:3001, domain set via $DOMAIN env var
- docker-compose: new caddy service (ports 80/443/443-udp), persistent
  caddy_data/caddy_config volumes; remove direct explorer port binding
- .env.example: replace EXPLORER_PORT with DOMAIN

Set DOMAIN=localhost for local dev (HTTP), DOMAIN=your.domain.com for
production (Caddy fetches Let's Encrypt cert automatically)

.env.example

@@ -20,8 +20,9 @@ WALLET_RPC_USER=
 WALLET_RPC_PASS=
 
 # ─── Explorer web ────────────────────────────────────────────────────────────
-# Host port the explorer is published on
-EXPLORER_PORT=3001
+# Public domain served by Caddy (gets a free Let's Encrypt cert automatically).
+# Use 'localhost' for local testing (HTTP only, no cert).
+DOMAIN=explorer.example.com
 # Bootswatch theme: Cerulean, Cosmo, Cyborg, Darkly, Flatly, Slate, Solar, ...
 EXPLORER_THEME=Purple