Listen on 0.0.0.0 by default and log LAN addresses

- New CANON_HOST (default 0.0.0.0; 127.0.0.1 keeps it on one machine)
- On start, log the http:// addresses other devices can use, skipping
  Docker/VM bridge interfaces
- docker-compose publishes 0.0.0.0:4242 explicitly
- README: opening Canon from other devices and firewall note

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -17,5 +17,6 @@ CANON_IGDB_CLIENT_SECRET=
 # CANON_TRUST_PROXY=true     # honor X-Forwarded-For / -Proto / -Host
 # CANON_SECURE_COOKIES=true  # always mark the session cookie Secure
 
+# CANON_HOST=0.0.0.0   # all network interfaces; 127.0.0.1 = this machine only
 # CANON_PORT=4242
 # CANON_LOG_LEVEL=debug