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/api.js

@@ -46,6 +46,7 @@ export const devicesApi = {
 }
 
 export const discoveryApi = {
-  scan: (data) => http.post('/discovery/scan', data),
-  ping: (ips)  => http.post('/discovery/ping', { ips }),
+  config: ()     => http.get('/discovery/config'),
+  scan:   (data) => http.post('/discovery/scan', data),
+  ping:   (ips)  => http.post('/discovery/ping', { ips }),
 }

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

frontend/src/i18n.js

@@ -118,7 +118,7 @@ const LANGS = {
     // DiscoveryModal
     autoDiscovery: 'Découverte automatique',
     dnsServer: 'Serveur DNS',
-    dnsHint: 'Le reverse DNS sera interrogé sur ce serveur pour résoudre les noms.',
+    dnsHint: 'Optionnel. Si renseigné, le reverse DNS (PTR) sera interrogé pour résoudre les noms d'hôtes. Configurez la valeur par défaut via DNS_SERVER dans .env.',
     vlansToScan: 'VLANs à scanner',
     vlansHint: 'Seuls les VLANs avec un sous-réseau CIDR configuré peuvent être scannés.',
     noCidrWarning: "Aucun VLAN n'a de CIDR configuré. Renseignez-les dans l'onglet VLANs.",
@@ -277,7 +277,7 @@ const LANGS = {
     subnetPlaceholder: 'e.g. 192.168.10.0/24',
     autoDiscovery: 'Auto discovery',
     dnsServer: 'DNS server',
-    dnsHint: 'Reverse DNS will be queried on this server to resolve hostnames.',
+    dnsHint: 'Optional. If set, reverse DNS (PTR) will be queried to resolve hostnames. Set the default via DNS_SERVER in .env.',
     vlansToScan: 'VLANs to scan',
     vlansHint: 'Only VLANs with a configured CIDR subnet can be scanned.',
     noCidrWarning: 'No VLAN has a CIDR configured. Set them in the Networks tab.',
@@ -435,7 +435,7 @@ const LANGS = {
     subnetPlaceholder: 'ej: 192.168.10.0/24',
     autoDiscovery: 'Descubrimiento automático',
     dnsServer: 'Servidor DNS',
-    dnsHint: 'El DNS inverso será consultado en este servidor para resolver nombres.',
+    dnsHint: 'Opcional. Si se indica, se consultará el DNS inverso (PTR) para resolver nombres de host. Configure el valor por defecto con DNS_SERVER en .env.',
     vlansToScan: 'VLANs a escanear',
     vlansHint: 'Solo los VLANs con subred CIDR configurada pueden escanearse.',
     noCidrWarning: 'Ninguna VLAN tiene CIDR configurado. Configúrelo en la pestaña Redes.',