v0.1.9 - Tavily claim verification fallback

Verification\n- Implements Tavily rating-claim verification behind TAVILY_API_KEY.\n- Preserves deterministic fixture fallback for CI, no-key runs, and Tavily failures.\n- Carries source_hash through Claim and audit trace surfaces.\n- Keeps final APPROVE/BLOCK/ESCALATE ownership in deterministic gate code.\n\n## Checks\n- pytest: 115 passed.\n- seed eval: 25/25.\n- frontend audit/lint/build: passed.\n- CI: https://github.com/vedantggwp/sentinel/actions/runs/26732367571\n- CodeQL: https://github.com/vedantggwp/sentinel/actions/runs/26732367585