feat(discovery): world discovery metadata and world_visits model with migration 0015

backend/app/models/world_visit.py

@@ -0,0 +1,27 @@
+"""World visit tracking model for discovery analytics.
+
+docs/planning/discovery.md (task b377353b8167).
+"""
+
+from sqlalchemy import Column, Integer, String, DateTime, ForeignKey, Index
+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, index=True)
+    user_id = Column(Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True, index=True)
+    anonymous_token = Column(String(255), nullable=True)
+    visited_at = Column(DateTime(timezone=True), server_default=func.now(), nullable=False, index=True)
+
+    __table_args__ = (
+        Index("ix_world_visits_world_visited_at", "world_id", "visited_at"),
+    )
+
+    world = relationship("World", back_populates="visits")
+    user = relationship("User")