Write README and dev docs

[bostdiek]

Description:
Create comprehensive documentation for multi-environment development workflow.

Developer Story:
As a developer, I want clear documentation so I can contribute effectively and understand the full development lifecycle.

Acceptance Criteria:
• README includes:

• Project overview and tech stack (uv, PostgreSQL, React, Tailwind)
• Quick start: make dev and make prod
• Development workflow and folder structure
• API versioning strategy and endpoint documentation
• Database migration commands (Alembic)
• Frontend/backend development commands
• Environment variable documentation
  • docs/ directory with:
• API_DESIGN.md (versioning strategy, response formats)
• DEPLOYMENT.md (production deployment guide)
• CONTRIBUTING.md (code style, testing, PR process)
  • Makefile with common commands:
• make dev, make prod, make test, make lint, make migrate