feat: add shared detection content specification v1

[egrexsec]

Summary

• add the shared DetLab Detection Content Specification v1 JSON Schema
• normalize rich DetLab detection YAML into the portable contract
• record canonical source hashes and generated-artifact provenance/staleness
• validate every authored detection YAML file
• add a Python contract CI job while preserving the existing static Next.js build
• document that rich DetLab YAML remains canonical and generated queries remain derived

Validation

• PYTHONPATH=service python -m unittest discover -s service/tests -p 'test_*.py' -v
• cd web && npm test
• cd web && npm run build
• git diff --check

Boundaries

• this PR adds the shared contract and adapter, not the conversion API or workbench controls
• no generated query is represented as production/live validated
• contract schema is byte-identical to the copy proposed in egrexsec/cybersecurity-playbook