feat: add optional COCO metric backend

[hyeokjun32]

Summary

• Added a metric backend layer for detection evaluation.
• Kept simplified mAP50 as the default backend and added explicit optional pycocotools backend support.
• Recorded metric backend metadata in evaluation JSON/report fixtures and docs.

Validation

• poetry run python3 -m pytest tests/test_metric_backends.py tests/test_evaluate_detection.py tests/test_validation_contracts.py tests/test_validation_demo_report.py -q
• poetry run python3 -m pytest -q

Impact

• Preserves lightweight local-first evaluation by default.
• Allows official COCO evaluator usage only when explicitly requested and installed.
• Avoids silent metric fallback when pycocotools is missing.

Notes

• No new production SaaS infrastructure was added.
• pycocotools remains optional and is not required for the default test suite.