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, ARRAY
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, ARRAY, Index
 from sqlalchemy.dialects.postgresql import ENUM
 from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
@@ -33,11 +33,11 @@ class World(Base):
     play_count = Column(Integer, nullable=False, server_default="0")
     last_played_at = Column(DateTime(timezone=True), nullable=True)
 
+    # GIN index on tags for efficient tag-based queries
+    __table_args__ = (
+        Index("ix_worlds_tags", "tags", postgresql_using="gin"),
+    )
+
     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},
-    )