feat(stage5): implement claim extraction — atomic verifiable claims from sources
- Claim model with provenance, evidence_span, attribution, claim_type
- Stage5Extractor: LLM-based atomic claim extraction from source content
- Never summarizes — always extracts atomic, verifiable claims
- Claims require evidence span (exact quote from source)
- Attribution per claim (who says what)
- Claim types: fact, opinion, prediction, recommendation, claim
- Confidence score 0.0–1.0 per claim
- Bounded concurrency, SSRF-safe, max content truncation
- REST API: GET/POST /research/{run_id}/claims
- 36 tests: parsing, edge cases, integration, validation
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