Document database schema management strategy after Alembic removal

[caitlon]

Context

PR #90 removed Alembic migrations from the project. The rationale was simplification for the thesis project scope. However, this needs documentation for future maintainers.

Problem Statement

Without migrations:

• Schema changes require manual database recreation
• Production deployments need documented process
• No rollback capability for schema changes
• Future contributors may be confused by lack of migrations

Proposed Solution

Document the chosen approach in api/README.md:

1. SQLModel's create_all() handles schema creation
2. For production: use Supabase's built-in migration tools or manual SQL
3. Document schema change process for future maintainers
4. Note trade-offs of current approach vs migrations

Acceptance Criteria

• Document schema management approach in api/README.md
• Add section on production database setup
• Note trade-offs of current approach vs migrations
• Include instructions for adding new models
• Document how to handle schema changes in production

Technical Notes

• SQLModel creates tables via SQLModel.metadata.create_all(engine)
• Supabase provides SQL editor for manual migrations
• For thesis scope, this approach is sufficient