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>

.env.example

@@ -7,3 +7,9 @@ RETENTION=30d
 # Web UI authentication (empty = disabled)
 AUTH_USER=
 AUTH_PASS=
+# Reverse DNS: show host names instead of IP addresses (on/off)
+RDNS=on
+# DNS server used for reverse lookups (e.g. your router: 192.168.1.1). Empty = system resolver
+DNS_SERVER=
+# Allow "Delete all logs" in Settings (true/false)
+ALLOW_PURGE=true