feat: deploy to Frostmoln production (#417)

Part of the dancefinder.unbound.se cutover to Frostmoln Production (infra: unboundsoftware/infra#1896, already applied).

- Replaces the AWS `deploy-prod` job (failing since CI left AWS) with `deploy-prod-frostmoln` (`deploy prod-frostmoln -n dancefinder`), as schemas did.
- Restricted securityContext (runAsNonRoot, drop ALL, seccomp RuntimeDefault) and `imagePullSecrets: oci-registry` (Platform registry).
- Listens on 8080 (`PORT`) as uid 65534. MAPS_API_KEY comes from the namespace's SecretStore (Frostmoln Secret `dancefinder`).

Merging deploys to Frostmoln only; DNS still points at AWS until the cutover.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01H6eVuR4Yt3m7TbDTDwyZJi
Reviewed-on: https://gitea.unbound.se/dancefinder/geo-service/pulls/417

k8s/secrets-prod-frostmoln.yaml

@@ -0,0 +1,21 @@
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: geo-service
+spec:
+  refreshInterval: 1h
+  # The namespace's store (infra frostmoln/production/apps.tf); MAPS_API_KEY
+  # lives in the Frostmoln Secret "dancefinder", next to the API's settings
+  # (infra frostmoln/production/scripts/seed-app.sh).
+  secretStoreRef:
+    name: frostmoln
+    kind: SecretStore
+  target:
+    name: geo-service
+    template:
+      engineVersion: v2
+      data:
+        MAPS_API_KEY: "{{ .MAPS_API_KEY }}"
+  dataFrom:
+  - extract:
+      key: dancefinder