[samperay]

Summary

• Add the local Prometheus monitoring specification, plan, and verification procedure.
• Start the existing Todo API and Prometheus together with Docker Compose. Prometheus scrapes GET /metrics every 15 seconds at todo-api:8000.
• Publish the API on 127.0.0.1:8000 and Prometheus on 127.0.0.1:9090. Keep Prometheus samples in the named volume prometheus-data. The API's SQLite file is not on a volume.
• Document the PromQL queries, the difference between an empty result and numeric 0, promtool validation, and the baseline-then-traffic verification procedure.
• Check the committed Compose file, Prometheus file, and README in pytest. Existing metric names and labels are unchanged. Grafana and alerting are not included.

CI results

Check	Result
Ruff Lint	Passed
Ruff Format	Passed
Mypy	Passed
Pytest	Passed
Coverage	97.39%

Checked locally before this pull request was published.