Add integration examples

[Bmowville]

Summary

• add an integration guide for reporting pipeline events into the API
• add copy-paste examples for Python jobs, dbt run results, Airflow, and GitHub Actions
• link the integration flow from the README
• add tests that keep the integration examples present and syntactically valid

Validation

• python -m ruff check .
• python -m mypy app
• python -m alembic upgrade head
• python -m pytest
• git diff --check
• ran examples/integrations/python_reporter.py against the local API