Research: Duplicate report detection offline fixtures

[codethor0]

Background

Future moderation tooling needs duplicate detection by normalized job URL and related metadata. Offline research and fixtures can proceed before schema work.

Scope

• Research duplicate detection rules aligned with docs/sheet-import-design.md
• Propose offline helper functions or test fixtures only
• Document findings in docs or issue comments

Acceptance criteria

• Research summary with proposed test cases
• No database writes
• No --apply implementation

Test / validation

• Offline unit tests only if code is added

Non-goals

• No Alembic migrations
• No production import
• No Sheet API integration

Privacy constraints

• Synthetic URLs and company names only; no live Sheet rows

Approval-required boundaries

• Schema and API implementation require project-owner approval (see GOVERNANCE.md)