feat: add index on configs.download_token for token-auth lookups

backend/alembic/versions/0012_config_add_download_token.py

@@ -31,7 +31,9 @@ def upgrade() -> None:
         )
     # Remove the DB-level default — ORM model provides Python-level default
     op.alter_column("configs", "download_token", server_default=None)
+    op.create_index("ix_configs_download_token", "configs", ["download_token"])
 
 
 def downgrade() -> None:
+    op.drop_index("ix_configs_download_token", table_name="configs")
     op.drop_column("configs", "download_token")