Add entity linking and co-reference resolution

[jzonthemtn]

Summary

PhEye detects entity spans but does not link them. For example, "Dr. Smith" and "John Smith" in the same document are treated as independent detections with no indication they refer to the same entity.

Proposal

Add co-reference resolution:

• Cluster mentions that refer to the same real-world entity within a document
• Assign a stable entity ID to each cluster
• Expose clusters in the API response alongside span detections

This enables consistent pseudonymization downstream and improves redaction completeness.

Acceptance Criteria

• PhEye resolves co-references (e.g., "John Smith... he... the patient") to the same entity
• Entity linking connects detected entities to canonical identifiers where possible
• Improves downstream redaction consistency (all references to the same person are redacted)
• Tests cover pronoun resolution and entity linking on representative documents
• Accuracy benchmarked with Philter Scope