fix: Add last_completed_partition column to migration_state table schema

The migration_state table was missing the last_completed_partition column
that was referenced in the migration update queries. This column tracks
which partition was last completed to enable accurate resume capability.

To apply this change to existing databases:
  ALTER TABLE migration_state ADD COLUMN last_completed_partition VARCHAR(255);

For new databases, the table will be created with the column automatically.

src/transformers/schema_transformer.py

@@ -148,7 +148,8 @@ CREATE TABLE IF NOT EXISTS migration_state (
     migration_started_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
     migration_completed_at TIMESTAMP,
     total_rows_migrated BIGINT DEFAULT 0,
-    status VARCHAR(32) DEFAULT 'pending'
+    status VARCHAR(32) DEFAULT 'pending',
+    last_completed_partition VARCHAR(255)
 );
 """
     return sql