Add Drizzle SQL migrations

## Summary
Schema uses `drizzle-kit push` only. The `migrations/` folder is empty — not production-safe.

## Tasks
- [ ] Generate initial migration from current schema
- [ ] Add `db:migrate` script
- [ ] Update `db:setup` to use migrations in CI/production
- [ ] Document migration workflow in CONTRIBUTING.md

## Files
- `apps/api/drizzle.config.ts`
- `apps/api/src/db/schema.ts`
- `apps/api/src/db/migrations/`
- `package.json`

Start by reading apps/api/drizzle.config.ts and apps/api/src/db/schema.ts, then inspect the existing db:setup and related scripts in package.json. Generate the initial migration in apps/api/src/db/migrations/, add db:migrate, update setup for CI and production, and document the workflow in CONTRIBUTING.md; done means all four checklist tasks are complete.

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