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

backend/app/models/world.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, ARRAY
 from sqlalchemy.dialects.postgresql import ENUM
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
@@ -27,5 +27,17 @@ class World(Base):
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
 
+    # Discovery metadata
+    category = Column(String(100), nullable=True, index=True)
+    tags = Column(ARRAY(String), nullable=True, server_default="{}")
+    play_count = Column(Integer, nullable=False, server_default="0")
+    last_played_at = Column(DateTime(timezone=True), nullable=True)
+
     owner = relationship("User", back_populates="worlds")
     assets = relationship("Asset", back_populates="world", cascade="all, delete-orphan")
+    visits = relationship("WorldVisit", back_populates="world", cascade="all, delete-orphan")
+
+    __table_args__ = (
+        # GIN index on tags for efficient tag-based queries
+        {"sqlite_autoincrement": False},
+    )