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>

internal/http/web.go

@@ -219,10 +219,11 @@ func (s *Server) routesWeb(mux *http.ServeMux) {
 		})
 	}))
 
+	rolePage("/messages", "messages.html", "Messages", "messages", "/static/js/messages.js", nil)
+
 	// Placeholders for areas built in later phases; replaced as they land.
 	placeholders := map[string]string{
-		"/messages": "Messages",
-		"/metrics":  "Metrics",
+		"/metrics": "Metrics",
 	}
 	for path, title := range placeholders {
 		mux.HandleFunc("GET "+path, s.page(func(w http.ResponseWriter, r *http.Request, u *domain.User) {