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/Actors/RemoteActorService.cs

@@ -48,6 +48,7 @@ namespace PrivaPub.Federation.Actors
 
 		public async Task<FetchedJson> FetchObject(string uri, CancellationToken token)
 		{
+			using var scope = HttpScope.Default("object");
 			var signer = await _localActors.GetInstanceActor(token);
 			var fetched = await Get(uri, signer, token);
 			if (fetched == default)
@@ -81,6 +82,7 @@ namespace PrivaPub.Federation.Actors
 			if (!MayFetch(actorUri))
 				return cached;
 
+			using var scope = HttpScope.For("actor");
 			using var fetched = await FetchObject(actorUri, token);
 			var actor = fetched == default ? default : ActorDocument.Parse(fetched.Root);
 			if (actor == default)
@@ -101,6 +103,7 @@ namespace PrivaPub.Federation.Actors
 			if (!MayFetch(keyId))
 				return cached;
 
+			using var scope = HttpScope.For("key");
 			using var fetched = await FetchObject(StripFragment(keyId), token);
 			if (fetched == default)
 				return cached;
@@ -129,6 +132,7 @@ namespace PrivaPub.Federation.Actors
 			if (parts is not { Length: 2 } || string.IsNullOrEmpty(parts[0]) || string.IsNullOrEmpty(parts[1]))
 				return default;
 
+			using var scope = HttpScope.For("webfinger");
 			var query = $"/.well-known/webfinger?resource={Uri.EscapeDataString($"acct:{parts[0]}@{parts[1]}")}";
 			using var fetched = await _http.GetJson($"https://{parts[1]}{query}", "application/jrd+json, application/json", sign: default, token)
 				?? (_options?.CurrentValue.AllowPlainHttp == true