feat(backend): add sample freight CRUD and link/unlink

Provide freight batch create/update/link/unlink, sample enrichment map, and protocol-save draft grouping without writing legacy transport rows.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_sample_freight.py

@@ -1,6 +1,7 @@
 """样本运费：运输方式字典解析与关联规则"""
 
 import unittest
+from decimal import Decimal
 from unittest.mock import MagicMock
 
 from app.services.sample_transport_mode import resolve_transport_mode_value
@@ -39,5 +40,26 @@ class TestResolveTransportMode(unittest.TestCase):
         db.add.assert_not_called()
 
 
+class TestFreightAmountValidation(unittest.TestCase):
+    def test_negative_rejected(self):
+        from app.crud.sample_freight import _parse_amount
+
+        with self.assertRaises(ValueError):
+            _parse_amount(-1)
+
+    def test_zero_ok(self):
+        from app.crud.sample_freight import _parse_amount
+
+        self.assertEqual(_parse_amount(0), Decimal("0"))
+
+    def test_empty_rejected(self):
+        from app.crud.sample_freight import _parse_amount
+
+        with self.assertRaises(ValueError):
+            _parse_amount(None)
+        with self.assertRaises(ValueError):
+            _parse_amount("")
+
+
 if __name__ == "__main__":
     unittest.main()