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/Indexes.cs

@@ -54,6 +54,12 @@ namespace PrivaPub.Infrastructure.Data
 			await Plain<Group>(token, g => g.InvitationCode);
 
 			await Plain<DmGroup>(token, g => g.ParticipantsKey);
+			await Plain<DmGroup>(token, g => g.LastPostId);
+			await DB.Default.Index<ConversationState>()
+				.Key(s => s.AvatarId, KeyType.Ascending)
+				.Key(s => s.ConversationId, KeyType.Ascending)
+				.Option(o => o.Unique = true)
+				.CreateAsync(token);
 			await Unique<GroupAnnouncement>(a => a.ActivityURI, Builders<GroupAnnouncement>.Filter.Type(a => a.ActivityURI, BsonType.String), token);
 
 			await Plain<Delivery>(token, d => d.DeliveredAt, d => d.AbandonedAt, d => d.NextAttemptAt);