WIP checkpoint: Enforce 18+ age gate at account registration

backend/app/models/user.py

@@ -1,4 +1,6 @@
 from datetime import date
+from typing import Optional
+
 from sqlalchemy import Column, Integer, String, Boolean, DateTime, Date
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
@@ -21,3 +23,15 @@ class User(Base):
     sessions = relationship("Session", back_populates="user", cascade="all, delete-orphan")
     worlds = relationship("World", back_populates="owner")
     assets = relationship("Asset", back_populates="owner")
+
+    @property
+    def is_adult(self) -> bool:
+        """Return True if the user is 18 or older based on date_of_birth."""
+        if self.date_of_birth is None:
+            return False
+        today = date.today()
+        age = today.year - self.date_of_birth.year
+        # Adjust if birthday hasn't occurred yet this year
+        if (today.month, today.day) < (self.date_of_birth.month, self.date_of_birth.day):
+            age -= 1
+        return age >= 18