Prevent duplicate inserts for canonical/reference rows

[dgee2]

Summary

Implement lookup-before-insert behavior for canonical/reference entities that should be reused instead of duplicated.

Scope

• Identify reference-row creation paths
• Add lookup-and-reuse behavior before insert
• Align normalization/comparison with database uniqueness semantics

Deliverables

• Repository/service support for reuse instead of duplicate creation
• Tests covering existing-row reuse behavior

Acceptance criteria

• Existing canonical rows are reused when equivalent input is provided
• Covered paths no longer attempt duplicate inserts
• Behavior remains client-transparent where appropriate

Testing

• Unit tests for lookup-before-insert behavior
• Integration tests proving existing canonical rows are reused

Dependencies

• Depends on the duplicate-handling policy sub-issue

Parent: #887