add: Infinity datasource plugin for actual-http-api, wired into Grafana

Installs yesoreyeram-infinity-datasource on the Grafana pod so it can call
Actual's own ActualQL run-query endpoint directly (group-by/sum over any
field), instead of being limited to the fixed counters
actual-budget-exporter publishes. The datasource connection itself (URL +
x-api-key) is added by hand in the Grafana UI, not provisioned here — keeps
the API key out of git without needing a dedicated K8s Secret for it.

platform/apps/kube-prometheus-stack/values.yaml

@@ -3,6 +3,13 @@ grafana:
     enabled: true
     storageClassName: nfs-delete
     size: 5Gi
+  # Installs the Infinity datasource plugin so Grafana can query a plain
+  # REST API (actual-http-api) directly — see README for wiring it up.
+  # The datasource itself (URL + x-api-key) is added by hand in the Grafana
+  # UI, not provisioned here — avoids a second hand-created Secret just for
+  # this one credential.
+  plugins:
+    - yesoreyeram-infinity-datasource
 
 prometheus:
   prometheusSpec: