- migrations/084_add_user_theme.sql: ALTER TABLE user ADD COLUMN theme TEXT NOT NULL DEFAULT 'system'
- Defensive backfill UPDATE for in-flight NULL rows on managed Postgres
- COMMENT ON COLUMN documents allowed values (light|dark|system)
- lib/auth.ts: adds theme additionalField with defaultValue 'system' after timezone
- session.user.theme now exposed via Better Auth same as session.user.timezone
