Conversations page, and know what is unread

/api/v1/conversations answered one page, never unread, its read endpoint
did nothing, and DELETE was missing; each conversation cost a query per
member. Now each conversation keeps its newest post (DmGroup.LastPostId,
set as posts arrive, learnt once by migration _013) and pages by it as
Mastodon does, and each persona's ConversationState holds what it read and
what it took off its list:

- unread when someone else wrote last, after what the persona read;
- read marks it so, and writing in a conversation reads it;
- DELETE takes it off the list until a newer message brings it back.

The list reads its states, newest posts, members and accounts in a few
queries per page.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub/Infrastructure/Data/Migrations/_013_conversations_know_their_newest_post.cs

@@ -0,0 +1,24 @@
+using MongoDB.Entities;
+
+using PrivaPub.Models.Group;
+
+using PostEntity = PrivaPub.Models.Post.Post;
+
+namespace PrivaPub.Infrastructure.Data.Migrations
+{
+	// the conversations list pages by each conversation's newest post (DmGroup.LastPostId), set from now on as posts
+	// arrive; the conversations made before learn theirs once
+	public class _013_conversations_know_their_newest_post : IMigration
+	{
+		public async Task UpgradeAsync()
+		{
+			foreach (var group in await DB.Default.Find<DmGroup>().Match(g => g.LastPostId == null).ExecuteAsync())
+			{
+				var newest = await DB.Default.Find<PostEntity>().Match(p => p.ConversationId == group.ID && !p.DeletedAt.HasValue)
+					.Sort(p => p.ID, Order.Descending).ExecuteFirstAsync();
+				if (newest != default)
+					await DB.Default.Update<DmGroup>().MatchID(group.ID).Modify(g => g.LastPostId, newest.ID).ExecuteAsync();
+			}
+		}
+	}
+}