M5: outbound requests, previews, the media proxy and served traffic

- HttpScope (AsyncLocal) tags each outbound request with a purpose and a trigger:
  - purpose is set by the caller: actor, key, object, webfinger, context, nodeinfo;
  - trigger is set by the job kind, by "verify" during inbox verification, or defaults
    to "request".
- FederationHttp records every JSON, media and stream fetch: status, time, bytes, hops,
  and an outcome of ok, refused or failed, with a reason: disallowed, remembered,
  bad-redirect, too-many-redirects, content-type, too-large, bad-json, private-address,
  timeout, network, or the status. A fetch a reader caused (trigger "request") is only
  counted per server per day.
- Link previews record a 'preview' event: card, no-card or failed.
- The media proxy counts cache hits.
- TrafficMeter counts the client API per endpoint group, method and status class. It
  counts our served documents (actor, outbox, collection, object, activity, licence,
  webfinger, nodeinfo) by kind, status and whether signed, per day and never per server,
  and never names a circle's collections.

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

PrivaPub/Federation/Inbox/InboxReceiver.cs

@@ -2,6 +2,7 @@ using PrivaPub.Federation.Actors;
 using PrivaPub.Federation.Moderation;
 using PrivaPub.Federation.Objects;
 using PrivaPub.Federation.Signing;
+using PrivaPub.Infrastructure.Http;
 using PrivaPub.Infrastructure.Jobs;
 using PrivaPub.Infrastructure.Statistics;
 using PrivaPub.Models.Federation;
@@ -72,6 +73,7 @@ namespace PrivaPub.Federation.Inbox
 
 		public async Task<InboxResult> Receive(HttpRequest request, LocalActor recipient, CancellationToken token)
 		{
+			using var scope = HttpScope.Triggered("verify");
 			var started = System.Diagnostics.Stopwatch.GetTimestamp();
 			var receipt = new Receipt { Inbox = recipient == default ? "shared" : "personal", Bytes = request.ContentLength };
 			InboxResult result = default;