WIP checkpoint: World discovery metadata + visits model & Alembic migration

backend/app/models/__init__.py

@@ -2,5 +2,6 @@ from app.models.user import User
 from app.models.session import Session
 from app.models.world import World
 from app.models.asset import Asset
+from app.models.world_visit import WorldVisit
 
-__all__ = ["User", "Session", "World", "Asset"]
+__all__ = ["User", "Session", "World", "Asset", "WorldVisit"]

backend/app/models/world_visit.py

@@ -0,0 +1,26 @@
+from sqlalchemy import Column, DateTime, ForeignKey, Index, Integer, String
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from app.database import Base
+
+
+class WorldVisit(Base):
+    __tablename__ = "world_visits"
+
+    id = Column(Integer, primary_key=True, index=True)
+    world_id = Column(
+        Integer, ForeignKey("worlds.id", ondelete="CASCADE"), nullable=False
+    )
+    user_id = Column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=True
+    )
+    anonymous_token = Column(String(255), nullable=True)
+    visited_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    __table_args__ = (
+        Index("ix_world_visits_world_id_visited_at", "world_id", "visited_at"),
+    )
+
+    world = relationship("World", back_populates="visits")
+    user = relationship("User")