Add operations overview endpoint

[Bmowville]

Summary

• add /api/v1/metrics/operations-overview for a dashboard-ready operator snapshot
• include service status, summary metrics, pipeline rollups, quality rollups, stale runs, and recommended actions
• add test coverage for the overview response
• add .env.example, .gitignore, and GitHub Actions CI
• document the operations overview endpoint in the README

Validation

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