Remove persisted reference simulation semantics

[tomvothecoder]

Description

• Removes persisted reference simulation state and run-config delta payloads from the backend and frontend.
• Keeps case_hash as per-run grouping metadata and simplifies case/browse/details flows around neutral compare behavior.
• Updates the migration, seed data, docs, and ingestion/assistant tests to match the new model.
• Closes [Enhancement]: Remove reference model and use case-hash-scoped compare #195

Checklist

• Code follows project style guidelines
• Self-reviewed code
• No new warnings
• Tests added or updated (if needed)
• All tests pass (locally and CI/CD)
• Documentation/comments updated (if needed)
• Breaking change noted (if applicable)

Deployment Notes (if any)

• Run the migration 20260604_120000_remove_reference_and_delta_fields with the backend deploy.
• Backend and frontend should deploy together because simulation API fields were removed.

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[tomvothecoder]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

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