Add Alembic migrations setup

[Orlando-Alvarez]

Summary

This pull request adds Alembic database migration support to Ticket-API.

Changes

• Added Alembic as a project dependency.
• Added Alembic configuration files.
• Configured Alembic to read the existing DATABASE_URL from the application settings.
• Connected Alembic to the existing SQLAlchemy Base.metadata.
• Added the initial migration for the current database models:
  • users
  • tickets
  • incidents
• Updated the README with basic migration commands.

Validation

• Confirmed existing tests pass with:

python -m pytest

• Confirmed the initial migration can be applied with:

python -m alembic upgrade head

• Confirmed the database schema includes the expected tables:
  • users
  • tickets
  • incidents
  • alembic_version

Notes

• This PR does not add CRUD endpoints.
• This PR does not add authentication.
• This PR does not add cloud deployment configuration.
• This PR does not run migrations automatically when the FastAPI app starts.