add: actual-budget-exporter app

Prometheus exporter for Actual Budget. actual_server itself stays in
homelab-docker-compose-prod (host docker-compose), exporter only scrapes it
over LAN and exposes /metrics via a ServiceMonitor. No HTTPRoute. Password
and budget ID come from a hand-created Secret, not committed.

apps/actual-budget-exporter/chart/templates/servicemonitor.yaml

@@ -0,0 +1,22 @@
+{{- if .Values.serviceMonitor.enabled }}
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: {{ .Release.Name }}
+  labels:
+    # kube-prometheus-stack's default serviceMonitorSelector only picks up
+    # ServiceMonitors carrying this label (release name of that Helm
+    # release, set by its ArgoCD Application name in cluster-platform).
+    release: kube-prometheus-stack
+spec:
+  namespaceSelector:
+    matchNames:
+      - {{ .Release.Namespace }}
+  selector:
+    matchLabels:
+      app: {{ .Release.Name }}
+  endpoints:
+    - port: metrics
+      path: /metrics
+      interval: {{ .Values.serviceMonitor.interval }}
+{{- end }}