feat: DNS_SERVER env var — pre-fills discovery UI, optional for scan

- DNS_SERVER env var configures the default DNS server for PTR lookups
- GET /api/discovery/config exposes it to the frontend
- DiscoveryModal fetches it on mount and pre-fills the field (editable)
- dns_server is now optional in ScanRequest (default empty string)
- PTR lookup is skipped when dns_server is empty — scan still proceeds
- Validator only runs when dns_server is non-empty
- .env.example, docker-compose.yml, READMEs (fr/en/es) updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -16,6 +16,7 @@ services:
       - SECRET_KEY=${SECRET_KEY}
       - INITIAL_ADMIN_PASSWORD=${INITIAL_ADMIN_PASSWORD}
       - ALLOWED_ORIGINS=${ALLOWED_ORIGINS:-*}
+      - DNS_SERVER=${DNS_SERVER:-}
     # ── Docker secret alternative for SECRET_KEY ─────────────────────────────
     # Instead of passing SECRET_KEY via environment, you can mount a secret file.
     # The backend reads data/secret_key.txt when SECRET_KEY env var is unset.