feat(desktop): implement workspace foundation + desktop-client skeleton

Plan 0 (workspaces): add workspaces + workspace_memberships schema, extend
JWT/Actor/claims with primary_workspace_id, seed default workspace per tenant,
thread workspace_id through tenant monitoring quota.

Plan A (desktop skeleton): new Electron app (apps/desktop-client) with main/
preload/renderer, shared Vue component package (packages/ui-shared), and server
surface — desktop client registration + token rotation + heartbeat, SSE task
event stream, desktop accounts/tasks/content handlers, publish job endpoint,
and supporting repositories, services, sqlc queries, and migrations.

Hard cutover per plan: remove browser-extension monitoring callback endpoints,
stub legacy media API in admin-web, and delete monitoring_callback_handler.go.

server/cmd/tenant-api/main.go

@@ -29,6 +29,7 @@ func main() {
 	defer workerCancel()
 
 	app.GenerationStreams.Run(workerCtx)
+	app.DesktopTaskStreams.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)