[samperay]

Summary

• Adds a self-paced observability handbook for the Todo API. These files were untracked on docs/observability-handbook and are now in one docs commit: 32 Markdown files, 1999 lines.
• The workbook has 10 modules and 40 classes, from observability basics through Prometheus, PromQL, Grafana, alerting, application metrics, Loki, OpenTelemetry, SRE practice, and advanced operations.
• Labs, 12 incident scenarios, a PromQL cookbook, a glossary, and a progress tracker sit beside the modules. No application or Compose code changed.

Previously untracked files

• tutorials/observability-handbook/README.md — how to use the handbook and the local Todo API assumptions
• modules/01-observability-fundamentals.md through modules/10-advanced-observability.md
• labs/00-lab-setup.md, labs/01-metrics-traffic.md, labs/02-alerting.md, labs/03-full-stack.md, labs/04-kubernetes-cloud.md
• scenarios/README.md and scenarios 01–12 (API down, HTTP 500s, slow responses, traffic, pool exhaustion, crashes, scrape config, logs, traces, alert storms, error budgets, and a full outage drill)
• reference/promql-cookbook.md, reference/glossary.md, and progress.md

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.