fix: 补充 __init__.py 导出符号，修复测试导入错误

- src/core/extraction/__init__.py: 导出 _image_to_base64
- src/core/matching/__init__.py: 导出所有内部函数（_build_invoice_summary, _build_payment_records, _invoices_to_records, _match, _match_by_filename, _match_one_to_many, _match_one_to_one, _relative_tolerance, _safe_float）
- src/infra/documents/__init__.py: 导出 INVOICE_LEVEL_COLUMNS, PAYMENT_RECORD_COLUMNS
- tests/test_extractor.py: 修正 monkeypatch 路径为模块级引用

src/infra/documents/__init__.py

@@ -10,6 +10,8 @@ from .consumable import (
 )
 from .invoice import (
     CACHE_DIR_NAME,
+    INVOICE_LEVEL_COLUMNS,
+    PAYMENT_RECORD_COLUMNS,
     classify_invoice_batch,
     load_csv,
     load_invoice_csv,
@@ -20,6 +22,8 @@ from .invoice import (
 
 __all__ = [
     "CACHE_DIR_NAME",
+    "INVOICE_LEVEL_COLUMNS",
+    "PAYMENT_RECORD_COLUMNS",
     "classify_invoice_batch",
     "load_csv",
     "load_invoice_csv",