Added pytest configuration and first 4 unit tests

app/models/db.py

@@ -2,7 +2,7 @@ from datetime import datetime, timezone
 import uuid
 from sqlalchemy import Column, String, JSON, DateTime, UniqueConstraint, ForeignKey, Boolean, Integer
 from sqlalchemy.dialects.postgresql import UUID
-from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import declarative_base
 from sqlalchemy.orm import relationship
 
 Base = declarative_base()