feat(protocols): conversations - thread_id and in_reply_to written at delivery, list and read APIs

A reply now joins the conversation of the message it answers (Message-ID,
In-Reply-To, References), falling back to the subject for replies with no
headers. /mail/threads lists a folder as conversations and /mail/thread/{id}
returns one. Replying from the webmail flags the answered message.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/protocols/cmd/lambdamail-protocols/main.go

@@ -209,6 +209,7 @@ func run(cfg config) {
 	if cfg.JwtSecret == "" {
 		log.Printf("JWT_SECRET is not set: the webmail mail API stays disabled")
 	}
+	webmailUC.SetConversations(postgres.NewConversations(pool))
 	router.SetMailAPI(webmailUC, cfg.JwtSecret)
 	// Folders the mailbox owner keeps for themselves, which the filing rules
 	// can then name as a destination.