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/base.py

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+# backend/models/base.py
+from datetime import datetime, timezone
+import uuid
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy import Column, DateTime, Boolean
+from sqlalchemy.dialects.postgresql import UUID
+
+Base = declarative_base()
+
+
+class TimestampMixin:
+    created_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), nullable=False)
+    updated_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc),
+                        onupdate=lambda: datetime.now(timezone.utc), nullable=False)
+
+
+class UUIDMixin:
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)

backend/app/models/user.py

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+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}>"