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>

frontend/src/components/DiscoveryModal.vue

@@ -173,7 +173,7 @@
 </template>
 
 <script setup>
-import { ref, computed } from 'vue'
+import { ref, computed, onMounted } from 'vue'
 import { devicesApi, discoveryApi } from '../api.js'
 import { t } from '../i18n.js'
 
@@ -194,6 +194,13 @@ const configError    = ref('')
 const importError    = ref('')
 const importing      = ref(false)
 
+onMounted(async () => {
+  try {
+    const resp = await discoveryApi.config()
+    dnsServer.value = resp.data.dns_server || ''
+  } catch { /* ignore — field stays empty */ }
+})
+
 const scanableVlans = computed(() => props.vlans.filter(v => v.cidr))
 
 const existingIps = computed(() => {
@@ -246,10 +253,6 @@ function toggleAll(e) {
 
 async function startScan() {
   configError.value = ''
-  if (!dnsServer.value.trim()) {
-    configError.value = t('dnsRequired')
-    return
-  }
   if (selectedVlanIds.value.length === 0) {
     configError.value = t('selectVlan')
     return