[monitoring] Put uk-payslip dashboard in Finance folder

Grafana can't auto-create the reserved 'General' folder ('A folder with
that name already exists'), which aborts the sidecar provisioner's walk
and drops every dashboard in that folder. Move uk-payslip to Finance so
it loads.

stacks/monitoring/modules/monitoring/grafana.tf

@@ -134,6 +134,7 @@ locals {
     # Applications
     "qbittorrent.json"           = "Applications"
     "realestate-crawler.json"    = "Applications"
+    "uk-payslip.json"            = "Finance"
   }
 }