feat(desktop): push publish tasks via AMQP topic dispatch WebSocket

Introduce a desktop.task.dispatch topic exchange with per-client routing
keys. Tenant-api publishes a task_available frame keyed by target client
ID when a publish job is created; every instance binds its own transient
queue and forwards to the matching WebSocket (/api/desktop/dispatch) it
owns. Desktop-client now prefers this push channel and only falls back
to HTTP /lease polling when the socket is down. Also drop admin-web
monitoring-plugin remnants and scope the publish modal account list to
publish platforms.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/cmd/tenant-api/main.go

@@ -30,6 +30,7 @@ func main() {
 
 	app.GenerationStreams.Run(workerCtx)
 	app.DesktopTaskStreams.Run(workerCtx)
+	app.DesktopDispatch.Run(workerCtx)
 
 	monitoringCallbackService := tenantapp.NewMonitoringCallbackService(app.DB, app.MonitoringDB, app.RabbitMQ, app.LLM, app.Logger)
 	tenantapp.NewMonitoringResultIngestWorker(monitoringCallbackService, app.RabbitMQ, app.Logger, app.Config.MonitoringWorkers).Start(workerCtx)