fix: delete soft-deleted conversations before dropping is_deleted column

Ensure any existing soft-deleted conversations are removed from the
database before dropping the is_deleted column in the migration.

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

@@ -19,6 +19,9 @@ depends_on = None
 
 def upgrade():
     # ### commands auto generated by Alembic - please adjust! ###
+    # Delete any conversations where is_deleted is true before dropping the column
+    op.execute("DELETE FROM conversations WHERE is_deleted = true")
+    
     with op.batch_alter_table('conversations', schema=None) as batch_op:
         batch_op.drop_column('is_deleted')