chore: implement migration drift detection, CI rules, and duplicate c…

[VictorEzenma]

…leanup

closes #423

Description

This PR implements automated drift detection and strict ownership rules for our database migrations to prevent schemas from silently diverging between backend/migrations/ and supabase/migrations/.

Key changes include:

• Defined Ownership Rules: Updated CONTRIBUTING.md to clearly define domain ownership (Core/Frontend vs. Backend) and establish a unified 5-step migration workflow.
• Added CI Check: Created a GitHub Actions workflow (.github/workflows/migration-drift-check.yml) that runs the drift detection script automatically on pull requests to block duplicates or conflicts.
• Duplicate Cleanup: Added a cleanup:migrations command to package.json and removed the 4 known redundant backend migrations (create_audit_logs.sql, create_renewal_tables.sql, create_team_invitations.sql, add_pause_columns.sql) to establish a clean slate for the CI.

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Related Issue

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Test Plan

• Tested locally
• Verified expected behavior
• No regressions introduced

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Screenshots (if applicable)

(N/A - Infrastructure and CI changes only)

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed

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Checklist

• Code builds successfully
• Tests pass
• Follows project conventions
• No sensitive data exposed

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