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/Models/Group/DmGroup.cs

@@ -11,6 +11,7 @@ namespace PrivaPub.Models.Group
 		public List<GroupMember> Members { get; set; } = new();
 		public string ConversationURI { get; set; }//context of the federated thread
 		public string ParticipantsKey { get; set; }
+		public string LastPostId { get; set; }//its newest post, which pages the conversations list (ConversationStates)
 
 		public DateTime CreationDate { get; set; } = DateTime.UtcNow;
 		public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;