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>

.env.example

@@ -57,3 +57,13 @@ BIND_ADDRESS=0.0.0.0
 #
 DOCKER_UID=1000
 DOCKER_GID=1000
+
+# ── DNS server for auto-discovery ────────────────────────────────────────────
+# IP address of the DNS server used for PTR (reverse DNS) lookups during
+# host discovery. Pre-fills the field in the discovery UI (user can override).
+# Leave empty to disable reverse DNS lookups by default.
+#
+# Examples:
+#   DNS_SERVER=192.168.1.1    (your router / local DNS)
+#   DNS_SERVER=               (disabled — no PTR lookups)
+DNS_SERVER=