Add database migrations with Alembic

[darshacharya]

Description

The app currently uses Base.metadata.create_all() to create tables on startup. This doesn't support schema changes without dropping and recreating the database.

Requirements

• Set up Alembic for database migrations
• Create an initial migration from the existing models
• Replace init_db() in main.py with Alembic's migration runner
• Document migration workflow in README

Tech Notes

• Current DB setup: backend/app/database.py
• Models: backend/app/models/
• Alembic needs async support — use alembic[async] or configure for async SQLAlchemy
• SQLite for dev, but migrations should be PostgreSQL-compatible

Acceptance Criteria

• alembic init structure exists in backend/
• Initial migration captures all existing models
• alembic upgrade head works on a fresh database
• alembic revision --autogenerate detects model changes
• README documents how to create and run migrations