refactor: extract backup table name as module constant

Share backup_table_name between upgrade() and downgrade() functions
to avoid duplication and ensure consistency.

api/migrations/versions/2025_09_02_2012-4f02b6704509_remove_unused_is_deleted_field_from_.py

@@ -16,11 +16,13 @@ down_revision = 'b95962a3885c'
 branch_labels = None
 depends_on = None
 
+# Backup table name for soft-deleted conversations
+backup_table_name = 'conversations_4f02b6704509_bak'
+
 
 def upgrade():
     # ### commands auto generated by Alembic - please adjust! ###
     # Create backup table for soft-deleted conversations
-    backup_table_name = 'conversations_4f02b6704509_bak'
     
     # Check if there are any soft-deleted conversations
     result = op.get_bind().execute(sa.text("SELECT COUNT(*) FROM conversations WHERE is_deleted = true"))
@@ -48,7 +50,6 @@ def downgrade():
         batch_op.add_column(sa.Column('is_deleted', sa.BOOLEAN(), server_default=sa.text('false'), autoincrement=False, nullable=False))
     
     # Restore soft-deleted conversations from backup table if it exists
-    backup_table_name = 'conversations_4f02b6704509_bak'
     
     # Check if backup table exists
     result = op.get_bind().execute(sa.text(f"""