[Phase 4] flux: add Prometheus ServiceMonitor manifests for all three API services

Add flux/{zip-enrichment,holidays,air-quality}/servicemonitor.yaml so
Prometheus (kube-prometheus-stack) can auto-discover and scrape each
API service once deployed.

Each ServiceMonitor:
- Selects pods via app.kubernetes.io/name label
- Scrapes /metrics on the http port every 30s
- Uses release: kube-prometheus-stack label to match Prometheus selector
- Lives in the same namespace as the API service

Add each servicemonitor.yaml to its kustomization.yaml resources list.
kustomize build flux/ passes with no errors.

Closes leeworks-agents/api-company#110

flux/zip-enrichment/kustomization.yaml

@@ -4,3 +4,4 @@ resources:
   - namespace.yaml
   - externalsecret.yaml
   - helmrelease.yaml
+  - servicemonitor.yaml

flux/zip-enrichment/servicemonitor.yaml

@@ -0,0 +1,15 @@
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: zip-enrichment
+  namespace: zip-enrichment
+  labels:
+    release: kube-prometheus-stack
+spec:
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: zip-enrichment
+  endpoints:
+    - port: http
+      path: /metrics
+      interval: 30s