feat: add user model and registration endpoint

backend/app/models/user.py

@@ -0,0 +1,19 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, String
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    username: Mapped[str] = mapped_column(String(32), unique=True, index=True)
+    password_hash: Mapped[str] = mapped_column(String(255))
+    email: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )