Add base and user models with essential fields and mixins

Created reusable `Base`, `UUIDMixin`, and `TimestampMixin` in `base.py`. Added a `User` model with attributes, relationships, and database configurations in `user.py` to establish a foundation for user-related features.

backend/app/models/user.py

@@ -0,0 +1,24 @@
+from sqlalchemy import Column, String, JSON, Boolean
+from sqlalchemy.orm import relationship
+from .base import Base, TimestampMixin, UUIDMixin
+
+
+class User(Base, UUIDMixin, TimestampMixin):
+    __tablename__ = 'users'
+
+    email = Column(String, unique=True, nullable=False, index=True)
+    password_hash = Column(String, nullable=False)
+    first_name = Column(String, nullable=False)
+    last_name = Column(String, nullable=False)
+    phone_number = Column(String)
+    is_active = Column(Boolean, default=True, nullable=False)
+    is_superuser = Column(Boolean, default=False, nullable=False)
+    preferences = Column(JSON)
+
+    # Add relationships
+    created_events = relationship("Event", back_populates="creator", foreign_keys="Event.created_by")
+    managed_events = relationship("EventManager", back_populates="user")
+    guest_profiles = relationship("Guest", back_populates="user")
+
+    def __repr__(self):
+        return f"<User {self.email}>"