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

@@ -0,0 +1,18 @@
+# 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)