Improve OpenRouter graph extraction quality

[jefferdr-rgb]

Summary

• add OpenRouter DeepSeek/Kimi backends and make OpenRouter DeepSeek the default when configured
• sanitize malformed LLM graph fragments before cache/build/export paths
• add graphify quality schema gate for generated graph JSON
• repair common schema defects like missing labels, missing edge relations/source files, and confience_score typo

Verification

• /tmp/graphify-pytest-venv/bin/python -m pytest tests/test_llm_backends.py tests/test_cache.py tests/test_build.py tests/test_export.py tests/test_quality.py tests/test_prs.py
• HOME=/tmp/graphify-test-home /tmp/graphify-pytest-venv/bin/python -m pytest
• /tmp/graphify-pytest-venv/bin/graphify quality /Users/davidjeffery/graphify-corpora/dex-reference/dex-system-full-openrouter-deepseek-20260518T235254Z/graphify-out/graph.json
• git diff --check origin/v8..HEAD