Upgrade Prometheus to v3.10.0 (#301)

## Summary
- Bump Prometheus from v3.9.1 to v3.10.0 in custom container Dockerfile
- v3.10.0 adds distroless Docker image variants, new PromQL `fill` operators, and performance improvements
- Dagger build tested locally — builds cleanly

## Remaining after merge
- Update `kustomization.yaml` newTag with the auto-built image tag
- Update `service-versions.yaml` (last-reviewed + current-version)
- ArgoCD sync

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

Reviewed-on: #301

containers/prometheus/Dockerfile

@@ -1,7 +1,7 @@
 # Prometheus monitoring system
 # Three-stage build: Web UI (Node), binaries (Go), runtime (Alpine)
 
-ARG CONTAINER_APP_VERSION=v3.9.1
+ARG CONTAINER_APP_VERSION=v3.10.0
 ARG PROMETHEUS_VERSION=${CONTAINER_APP_VERSION}
 
 FROM node:22-alpine AS ui-build