Resolve host IPs through DNS, add purge, French date format by default

- Reverse DNS (cached PTR lookups): IP hosts are stored with their name
  in 'host' and the IP in 'host_ip'; older IP-only logs are resolved on
  display and the host filter shows 'name (IP)'. RDNS / DNS_SERVER env.
- Settings > Danger zone: delete all logs (type PURGE to confirm) through
  VictoriaLogs /delete/run_task; -delete.enable added to docker-compose.
  ALLOW_PURGE env to disable it.
- Remove the custom YYYY-MM-DD - HH:MM:SS:mmm format; default is now the
  usual French display DD/MM/YYYY HH:MM:SS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

syslog.go

@@ -34,6 +34,7 @@ type Entry struct {
 	Message  string
 	Source   string // sender IP address
 	Proto    string // udp or tcp
+	HostIP   string // original host value when it was an IP resolved through DNS
 }
 
 // Record returns the entry in the shape stored in VictoriaLogs and returned by the API.
@@ -56,6 +57,9 @@ func (e *Entry) Record() map[string]string {
 	if e.SD != "" {
 		r["sd"] = e.SD
 	}
+	if e.HostIP != "" {
+		r["host_ip"] = e.HostIP
+	}
 	return r
 }