Make codebase SQLite compatible

[marckohlbrugge]

Summary

• Replace jsonb with json in migrations (works on both PostgreSQL and SQLite)
• Replace PostgreSQL enum with string column (Rails enum handles validation in model)
• Replace uuid with gen_random_uuid() default with string (model generates UUID)
• Add conversion migration for existing PostgreSQL deployments (checks column types first, safe for fresh installs)

Test plan

• Ran migrations on existing PostgreSQL database
• Deployed to production via Kamal
• Test fresh SQLite install (swap pg gem for sqlite3, update database.yml)

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