Refactor: Unify analysis and created_by_analysis fields in Relationship model

[JSv4]

Background

The Relationship model currently has two separate fields for tracking analysis relationships:

• analysis (ForeignKey) - Organizational/filtering field (which mode relationship appears in)
• created_by_analysis (ForeignKey) - Privacy/permissions field (who can see it)

This dual-field design mirrors the same architectural pattern found in the Annotation model (see #495), introducing the same complexity and potential security concerns.

Current State (Option 1 - Implemented)

We've implemented validation to enforce consistency between these fields as a temporary mitigation. See opencontractserver/annotations/models.py:275-305.

Additionally, relationships can now be private to extracts via created_by_extract field with mutual exclusivity enforced.

Proposed Solution (Option 2)

Remove the analysis field entirely and use only created_by_analysis for both purposes:

• Simpler mental model
• Reduced validation complexity
• Better performance (one less field to index)
• Clearer semantics
• Consistency with Annotation model refactoring

Documentation

Full architectural analysis and migration strategy documented in:
docs/architecture/TODO_unify_relationship_analysis_fields.md

Related Work

• Refactor: Unify analysis and created_by_analysis fields in Annotation model #495 - Parallel refactoring effort for Annotation model
• Should be coordinated with annotation unification (do both together)

Priority

Medium - This is a refactoring to improve architecture, not a critical bug.

Effort

Medium - Requires data migration, code updates across multiple files, and thorough testing.