P6: quote posts, received and sent (FEP-044f and the older keys)

- Received quotes: read from `quote`, `quoteUrl`, `quoteUri`, `_misskey_quote` or a FEP-e232 Link tag; the quoted post
  is fetched once; a quoteAuthorization stamp is verified field by field on the quoted author's origin; a consent
  quote without a stamp is pending; an older-key quote of a public post is shown; Delete of a stamp revokes. Counts
  and a `quote` notification follow the accepted state. The `quote-inline` fallback survives sanitising and is removed
  from content when the real quote is shown.
- Personas quote through `quoted_status_id`: posts that state a quote policy get a QuoteRequest and stay pending until
  an Accept brings a stamp we can verify, then an Update adds quoteAuthorization; posts that state none are quoted
  the older way, without `quote`; another persona's posts cannot be quoted yet (we issue no stamps). Quoting posts
  are delivered to the quoted author too.
- Mastodon API: Status.quote (with the quoted status one level deep), quotes_count, quote_approval from the remote
  policy, GET /api/v1/statuses/:id/quotes, `quote` notifications, and api_versions.mastodon = 7.

Checked live: GoToSocial's author-only quote policy is respected.

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

PrivaPub/Federation/Rendering/ActivityPubRenderer.cs

@@ -39,6 +39,15 @@ namespace PrivaPub.Federation.Rendering
 				["blurhash"] = "toot:blurhash",
 				["votersCount"] = "toot:votersCount",
 				["Emoji"] = "toot:Emoji",
+				["quote"] = new JsonObject { ["@id"] = "https://w3id.org/fep/044f#quote", ["@type"] = "@id" },
+				["quoteAuthorization"] = new JsonObject { ["@id"] = "https://w3id.org/fep/044f#quoteAuthorization", ["@type"] = "@id" },
+				["QuoteRequest"] = "https://w3id.org/fep/044f#QuoteRequest",
+				["QuoteAuthorization"] = "https://w3id.org/fep/044f#QuoteAuthorization",
+				["misskey"] = "https://misskey-hub.net/ns#",
+				["_misskey_quote"] = "misskey:_misskey_quote",
+				["fedibird"] = "http://fedibird.com/ns#",
+				["quoteUri"] = "fedibird:quoteUri",
+				["quoteUrl"] = "as:quoteUrl",
 				["litepub"] = "http://litepub.social/ns#",
 				["EmojiReact"] = "litepub:EmojiReact",
 				["focalPoint"] = new JsonObject { ["@container"] = "@list", ["@id"] = "toot:focalPoint" },
@@ -160,6 +169,47 @@ namespace PrivaPub.Federation.Rendering
 			return note;
 		}
 
+		public static JsonObject UpdateOf(PostEntity post, LocalActor author, string reason)
+		{
+			var note = post.Visibility == PostVisibility.Direct
+				? DirectNote(post, author, Array.Empty<(string, string)>(), post.ContextURI)
+				: Note(post, author, default, post.InReplyToURI);
+			note["to"] = new JsonArray(post.To.Select(t => (JsonNode)t).ToArray());
+			note["cc"] = new JsonArray(post.Cc.Select(c => (JsonNode)c).ToArray());
+			return new JsonObject
+			{
+				["@context"] = Context(),
+				["id"] = author.ActivityUri($"update-{post.ID}-{reason}"),
+				["type"] = "Update",
+				["actor"] = author.Uri,
+				["to"] = note["to"]!.DeepClone(),
+				["cc"] = note["cc"]!.DeepClone(),
+				["object"] = note
+			};
+		}
+
+		static void AddQuote(JsonObject note, PostEntity post, ref string content)
+		{
+			if (string.IsNullOrEmpty(post.QuoteURI))
+				return;
+			var link = WebUtility.HtmlEncode(post.QuoteURI);
+			content += $"<p class=\"quote-inline\">RE: <a href=\"{link}\">{link}</a></p>";
+			if (post.QuoteByConsent)
+				note["quote"] = post.QuoteURI;
+			if (!string.IsNullOrEmpty(post.QuoteAuthorizationURI))
+				note["quoteAuthorization"] = post.QuoteAuthorizationURI;
+			note["_misskey_quote"] = post.QuoteURI;
+			note["quoteUri"] = post.QuoteURI;
+			note["quoteUrl"] = post.QuoteURI;
+			(note["tag"] as JsonArray)?.Add(new JsonObject
+			{
+				["type"] = "Link",
+				["mediaType"] = "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"",
+				["href"] = post.QuoteURI,
+				["name"] = $"RE: {post.QuoteURI}"
+			});
+		}
+
 		static void AddPoll(JsonObject note, PostPoll poll)
 		{
 			note["type"] = "Question";
@@ -211,6 +261,8 @@ namespace PrivaPub.Federation.Rendering
 					}))
 					.ToArray())
 			};
+			AddQuote(note, post, ref content);
+			note["content"] = content;
 			if (!string.IsNullOrEmpty(post.Language))
 				note["contentMap"] = new JsonObject { [post.Language] = content };
 			if (post.Poll is { } poll)