fix: migration to convert user_id text → uuid with FK to auth.users

From the PR description

Summary

• Adds backend/migrations/20260513_userid_text_to_uuid.sql - a transactional migration that converts user_id text columns to uuid and adds REFERENCES auth.users(id) FK constraints on all 9 affected tables
• workflows.user_id uses ON DELETE SET NULL (not CASCADE) to preserve shared workflows when the owner account is deleted
• Provides a pre-flight check query (in comments) to identify any non-UUID values before applying
• Adds rollback script 20260513_userid_text_to_uuid.rollback.sql

Closes #104 Closes #114

Affected tables

projects, project_subfolders, documents, chats, tabular_reviews, tabular_review_chats, workflows, hidden_workflows, workflow_shares

Changes

• backend/migrations/20260513_userid_text_to_uuid.sql - the migration (wrapped in BEGIN/COMMIT)
• backend/migrations/20260513_userid_text_to_uuid.rollback.sql - rollback script
• backend/src/lib/__tests__/useridMigration.test.ts - 5 static analysis tests
• backend/package.json - "test": "vitest run" script added

⚠️ Apply instructions

1. Run the pre-flight check query from the migration comments - confirm zero rows returned
2. Apply via Supabase dashboard SQL editor or psql
3. Verify with \d public.projects that user_id shows type uuid and a FK constraint

Test plan

• Unit tests added and passing (5/5)
• Backend build passes