fix: treat VS as a string on read and write

Go stringified sheet cells with fmt.Sprint, so a numeric VS read with
UNFORMATTED_VALUE was rendered as 2.609767878e+09. Text columns of the
payments sheet now go through sheets.CellString, which never uses
exponent form; bank inference uses the same helper.

Both sync implementations now append VS as a text cell so Sheets keeps
it a string and leading zeros survive. Existing rows are not rewritten.

Python already read VS as a digit string; a regression test pins that.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

legacy/tests/test_match_payments.py

@@ -1,6 +1,7 @@
 import unittest
+from unittest.mock import MagicMock, patch
 
-from scripts.match_payments import canonical_member_key, reconcile
+from scripts.match_payments import canonical_member_key, fetch_sheet_data, reconcile
 
 
 class TestCanonicalMemberKey(unittest.TestCase):
@@ -67,3 +68,23 @@ class TestReconcileTolerantPersonMatching(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class TestFetchSheetDataVS(unittest.TestCase):
+    """Numeric VS cells arrive as floats (UNFORMATTED_VALUE) and must stay plain digit strings."""
+
+    def _fetch(self, vs_cell):
+        service = MagicMock()
+        service.spreadsheets().values().get().execute.return_value = {"values": [
+            ["Date", "Amount", "manual fix", "Person", "Purpose", "Inferred Amount",
+             "Sender", "VS", "Message", "Bank ID", "Sync ID"],
+            ["2026-04-15", 750, "", "", "", "", "Test Sender", vs_cell, "fee", "", "abc"],
+        ]}
+        with patch("scripts.match_payments.get_sheets_service", return_value=service):
+            return fetch_sheet_data("sheet-id", "creds.json")
+
+    def test_numeric_vs_is_plain_digit_string(self):
+        self.assertEqual(self._fetch(2609767878.0)[0]["vs"], "2609767878")
+
+    def test_string_vs_is_kept(self):
+        self.assertEqual(self._fetch("0308")[0]["vs"], "0308")