Add API key ingestion auth

[Bmowville]

Summary

• add optional API-key protection for pipeline ingestion write endpoints
• keep dashboard and metrics reads open while requiring X-DataOps-API-Key when ingestion keys are configured
• update Python, dbt, Airflow, and GitHub Actions examples with API key support
• document local/demo behavior, deployed service behavior, and key rotation setup

Validation

• python -m ruff check .
• python -m mypy app
• python -m alembic upgrade head
• python -m pytest
• git diff --check