Bump Flask backend resource limits
## What
Increase `monitoring_flask_backend` resources so the Flask API has enough CPU and memory for real customer load.
## Scope
- Docker Compose: CPU `0.1` -> `0.5`, memory `192MiB` -> `1024MiB`.
- Helm: add explicit Flask resources with CPU request/limit `500m`, memory request `256Mi`, memory limit `1Gi`.
## Validation
Covered by !238.
issue