feat: messaging with conversations, rich text, attachments, typing, unread (phase 9)

- conversations: dm (unique pair, find-or-create), group (titled), project
  (customer-bound); participant-only access
- messages: server-sanitized rich text, attachments referencing uploads
  from the generic POST /api/v1/files endpoint (rate limited, MIME sniffed)
- unread counts via aggregation; mark-read pushes readBy receipts
- chat files served only to conversation participants (or uploader)
- @mentions in chat notify the mentioned participant
- typing indicator: client WS event relayed to other participants
- WS targeted delivery (SendTo) + 15s polling fallback
- two-pane messages UI: conversation list with unread badges, composer
  (contenteditable + formatting buttons), drag & drop upload, inline image
  previews with lightbox, new-conversation dialog with user search

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

cmd/app/main.go

@@ -86,6 +86,8 @@ func run() error {
 	})
 	srv.SetWSHandler(hub.Handle)
 	srv.SetPublishFn(hub.Broadcast)
+	srv.SetSendTo(hub.SendTo)
+	hub.SetInbound(srv.HandleInboundWS)
 
 	// Atomizer client honoring the admin base-URL override per call.
 	atomClient := atomize.New(func() string {