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>

backend/routers/discovery.py

@@ -1,5 +1,6 @@
 import errno
 import ipaddress
+import os
 import socket
 import subprocess
 import time
@@ -16,6 +17,8 @@ router = APIRouter()
 MAX_HOSTS_PER_TARGET = 1024   # refuse les /21 et plus larges
 MAX_HOSTS_TOTAL      = 4096   # cap global sur l'ensemble des targets
 
+_ENV_DNS = os.environ.get("DNS_SERVER", "").strip()
+
 
 class ScanTarget(BaseModel):
     vlan_id: int
@@ -23,20 +26,26 @@ class ScanTarget(BaseModel):
 
 
 class ScanRequest(BaseModel):
-    dns_server: str
+    dns_server: str = ""
     targets: list[ScanTarget]
     tcp_check: bool = False
 
     @field_validator("dns_server")
     @classmethod
     def _dns_server(cls, v: str) -> str:
-        try:
-            ipaddress.ip_address(v)
-        except ValueError:
-            raise ValueError(f"dns_server must be a valid IP address, got: {v!r}")
+        if v:
+            try:
+                ipaddress.ip_address(v)
+            except ValueError:
+                raise ValueError(f"dns_server must be a valid IP address, got: {v!r}")
         return v
 
 
+@router.get("/config")
+def get_config():
+    return {"dns_server": _ENV_DNS}
+
+
 class DiscoveredHost(BaseModel):
     ip: str
     hostname: Optional[str] = None
@@ -109,7 +118,7 @@ def _scan_one(ip: str, dns_server: str, vlan_id: int, cidr: str, tcp_check: bool
         return None
     if tcp_check and not _tcp_check(ip):
         return None
-    hostname = _ptr_lookup(ip, dns_server)
+    hostname = _ptr_lookup(ip, dns_server) if dns_server else None
     return DiscoveredHost(ip=ip, hostname=hostname, vlan_id=vlan_id, cidr=cidr)