Media: uploads stripped of metadata, attachments both ways, a remote media proxy

- /api/v1/media and /api/v2/media (and GET/PUT /api/v1/media/:id):
  images go through libvips (NetVips, its native build bundled):
  autorotated, every kind of metadata dropped (EXIF, GPS, XMP, IPTC,
  comments), capped at 4096 px, with a 640 px preview and a blurhash
  (own encoder, the reference algorithm); animated GIFs are re-encoded;
  video and audio are remuxed by ffmpeg with -map_metadata -1, never
  re-encoded, and a video gets a still preview. Files get random names
  under /var/lib/privapub/media, outside the web root deploys replace, and
  are served at /media/files with nosniff and a sandbox CSP.
- media_ids on create and edit (four at most, the persona's own, each used
  once); notes carry them as Document attachments with alt text, blurhash,
  focalPoint and size; inbound attachments were already kept.
- avatar and header uploads in update_credentials, cropped to 400x400 and
  1500x500, federated with Update{Person}.
- Remote media reaches clients only through /media/proxy/{hmac}/{url},
  fetched by the guarded client (no SVG, 40 MB cap) and cached outside the
  served root, trimmed to 5 GB; foreign avatars and headers use it too, so
  a client never contacts another server.
- MediaJanitor deletes uploads left unattached for a day.
- nginx accepts 100 MB bodies on the upload endpoints only (applied on Max).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -170,6 +170,28 @@ namespace PrivaPub.Federation.Rendering
 			};
 			if (!string.IsNullOrEmpty(post.Language))
 				note["contentMap"] = new JsonObject { [post.Language] = content };
+			var attachments = post.Media.Where(m => !string.IsNullOrEmpty(m.URL)).Select(m =>
+			{
+				var attachment = new JsonObject
+				{
+					["type"] = "Document",
+					["mediaType"] = m.ContentType,
+					["url"] = m.URL,
+					["name"] = m.Description
+				};
+				if (!string.IsNullOrEmpty(m.Blurhash))
+					attachment["blurhash"] = m.Blurhash;
+				if (m.Focus is { Length: 2 })
+					attachment["focalPoint"] = new JsonArray(m.Focus[0], m.Focus[1]);
+				if (m.Width.HasValue && m.Height.HasValue)
+				{
+					attachment["width"] = m.Width;
+					attachment["height"] = m.Height;
+				}
+				return (JsonNode)attachment;
+			}).ToArray();
+			if (attachments.Length > 0)
+				note["attachment"] = new JsonArray(attachments);
 			if (!string.IsNullOrEmpty(post.Title))
 				note["name"] = post.Title;
 			var summary = post.SpoilerText ?? (post.HasContentWarning ? post.Title ?? ContentWarning : default);