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/Api/Mastodon/Controllers/MediaController.cs

@@ -1,3 +1,4 @@
+using PrivaPub.Infrastructure.Statistics;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 
@@ -18,10 +19,13 @@ namespace PrivaPub.Api.Mastodon.Controllers
 		readonly IMediaService _media;
 		readonly IMediaProxy _proxy;
 
-		public MediaController(IMediaService media, IMediaProxy proxy)
+		readonly IInteractionLedger _ledger;
+
+		public MediaController(IMediaService media, IMediaProxy proxy, IInteractionLedger ledger = default)
 		{
 			_media = media;
 			_proxy = proxy;
+			_ledger = ledger;
 		}
 
 		[HttpPost("/api/v1/media"), HttpPost("/api/v2/media"), Scope("write:media"), RequestSizeLimit(UploadLimit),
@@ -67,7 +71,10 @@ namespace PrivaPub.Api.Mastodon.Controllers
 			Response.Headers["Content-Security-Policy"] = "default-src 'none'; sandbox";
 			Response.Headers["Cache-Control"] = "public, max-age=604800";
 			if (_proxy.Cached(url) is { Path: not null } cached)
+			{
+				_ledger?.Count(Interactions.HostOf(url), "media:hit");
 				return PhysicalFile(cached.Path, cached.ContentType, enableRangeProcessing: true);
+			}
 			if (Request.Headers.Range.Count == 0)
 			{
 				var (path, contentType) = await _proxy.Fetch(signature, encoded, token);