CDNs found by themselves, and servers followed through time

PrivaPub now finds CDNs three ways, best first: the address ranges the
CDNs publish (Cloudflare, Fastly, Amazon CloudFront, Bunny, Gcore,
Imperva), downloaded daily by CdnUpdater and kept in CdnRangeSet; the
CDN's fingerprint in the responses it already gets from a server
(EdgeHintsHandler on the federation client); and the networks that carry
only a CDN. The fixed ASN list is gone; ASNs shared with plain hosting
(AWS, DataPacket) no longer hide a server. A server's Geo records the
CDN, its domain and how it was found, and weekly snapshots now keep the
city and coordinates too.

Servers through time (ServerPlaces): /instances/:host/history lists a
server's weekly snapshots, a CDN-fronted server's geo names the CDN's
domain and where the server was before it (before_cdn), and
/api/privapub/v1/cdns and /cdns/:domain group servers by CDN with week
by week who joined and who left. Owner decisions recorded in ROADMAP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Infrastructure/Http/EdgeHints.cs

@@ -0,0 +1,55 @@
+using PrivaPub.Infrastructure.Geo;
+
+using System.Collections.Concurrent;
+
+namespace PrivaPub.Infrastructure.Http
+{
+	public interface IEdgeHints
+	{
+		void Remember(string host, CdnProvider cdn);
+		CdnProvider Of(string host);
+	}
+
+	// The CDN each remote host's responses came through, as their headers tell it (CdnCatalog fingerprints), for the
+	// weekly description of the server. Like ConnectedAddresses, memory only: what a host answers is never stored per
+	// request.
+	public class EdgeHints : IEdgeHints
+	{
+		const int MaxHosts = 50_000;
+
+		readonly ConcurrentDictionary<string, CdnProvider> _hints = new(StringComparer.OrdinalIgnoreCase);
+
+		public void Remember(string host, CdnProvider cdn)
+		{
+			if (string.IsNullOrEmpty(host))
+				return;
+			if (cdn == default)
+			{
+				_hints.TryRemove(host, out _);
+				return;
+			}
+			if (_hints.Count >= MaxHosts)
+				_hints.Clear();
+			_hints[host] = cdn;
+		}
+
+		public CdnProvider Of(string host) => host != default && _hints.TryGetValue(host, out var cdn) ? cdn : default;
+	}
+
+	// On the federation client: reads each response's CDN fingerprint for its host. A response without one forgets the
+	// host's hint, so a server that leaves its CDN is seen as itself again.
+	public class EdgeHintsHandler : DelegatingHandler
+	{
+		readonly IEdgeHints _hints;
+
+		public EdgeHintsHandler(IEdgeHints hints) => _hints = hints;
+
+		protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+		{
+			var response = await base.SendAsync(request, cancellationToken);
+			if (request.RequestUri is { } uri && (int)response.StatusCode is >= 200 and < 500)
+				_hints.Remember(uri.Host, CdnCatalog.FromResponse(response));
+			return response;
+		}
+	}
+}