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>

README.md

@@ -100,6 +100,7 @@ All configuration is via environment variables. See `.env.example` for the full
 | `INITIAL_ADMIN_PASSWORD` | _(empty)_ | Bootstrap admin password. If unset, `admin/admin` is used with forced change. |
 | `ALLOWED_ORIGINS` | `*` | CORS allowed origins (comma-separated). Set to your domain in production. |
 | `BIND_ADDRESS` | `0.0.0.0` | IP address to listen on. Set to the interface facing the reverse proxy. |
+| `DNS_SERVER` | _(empty)_ | DNS server for PTR lookups during auto-discovery. Pre-fills the field in the UI (overridable per scan). Leave empty to disable reverse DNS. |
 | `DOCKER_UID` / `DOCKER_GID` | `1000` | UID/GID for the backend process. Must match the host user owning `./db_data/`. |
 
 ```bash