Add multi-reviewer agreement and adjudication workflow

[jzonthemtn]

Summary

Arbiter has no inter-annotator agreement metrics or workflow for resolving disagreements between reviewers. In regulated environments, multiple reviewers may need to review the same document.

Proposal

Add multi-reviewer support:

• Assign the same document to multiple reviewers
• Calculate inter-annotator agreement (Cohen's kappa or similar)
• Adjudication workflow for resolving disagreements
• Track agreement metrics over time for quality assurance

Acceptance Criteria

• Multiple reviewers can be assigned to the same document
• Agreement metrics computed (inter-annotator agreement / Cohen's kappa)
• Disagreements flagged for adjudication by a senior reviewer
• Adjudication decision is final and logged in the audit trail
• Tests cover agreement calculation and adjudication workflow