Add SQLAlchemy models and database session setup

[Orlando-Alvarez]

Summary

This pull request adds the initial database foundation for Ticket-API.

Changes

• Added SQLAlchemy and PostgreSQL driver dependencies.
• Added SQLAlchemy DeclarativeBase.
• Added initial domain models:
  • User
  • Ticket
  • Incident
• Added model registration tests.
• Added database URL configuration.
• Added SQLAlchemy engine, SessionLocal, and get_db dependency.
• Added database configuration tests.

Validation

• Confirmed all tests pass locally with:

python -m pytest

• Confirmed GitHub Actions CI passes successfully.

Notes

• This PR does not connect to a real PostgreSQL database yet.
• This PR does not add CRUD endpoints, authentication, Alembic, Docker, or deployment configuration.