M9 (first part): the means to locate a server

- IConnectedAddresses remembers which address each host's last connection reached, set in
  SafeHttpHandlerFactory's connect callback. That is once per pooled connection, with no
  second DNS lookup, and the address is never stored.
- IGeoLocator / DbIpLocator reads the offline DB-IP Lite city and ASN databases (MaxMind
  .mmdb, via MaxMind.Db). It maps memory, swaps to new files within ten minutes, rounds
  coordinates to one decimal, never looks up a private address, and answers nothing
  when the files are missing. CdnNetworks names the CDNs whose edge addresses say nothing
  about where a server is.
- deploy/max/geo-update.sh fetches this or last month's databases, checks them and swaps
  them in atomically. The privapub-geo timer runs it monthly as www-data, and setup.sh
  installs the directory, the script, the units and a first download.

Describing servers will use these in M8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ELjqpznMFMNrJoJUj6K5p2

PrivaPub/Infrastructure/Http/ConnectedAddresses.cs

@@ -0,0 +1,29 @@
+using System.Collections.Concurrent;
+using System.Net;
+
+namespace PrivaPub.Infrastructure.Http
+{
+	public interface IConnectedAddresses
+	{
+		void Remember(string host, IPAddress address);
+		IPAddress Of(string host);
+	}
+
+	public class ConnectedAddresses : IConnectedAddresses
+	{
+		const int MaxHosts = 50_000;
+
+		readonly ConcurrentDictionary<string, IPAddress> _addresses = new(StringComparer.OrdinalIgnoreCase);
+
+		public void Remember(string host, IPAddress address)
+		{
+			if (string.IsNullOrEmpty(host) || address == default)
+				return;
+			if (_addresses.Count >= MaxHosts)
+				_addresses.Clear();
+			_addresses[host] = address.IsIPv4MappedToIPv6 ? address.MapToIPv4() : address;
+		}
+
+		public IPAddress Of(string host) => host != default && _addresses.TryGetValue(host, out var address) ? address : default;
+	}
+}