feat(py): M5.4 fix #2 — add vs and sync_id to payments tx projection

Python's fetch_sheet_data read 9 sheet columns but skipped VS and
Sync ID, causing make parity to report extra fields on every raw
payment row emitted by the Go backend. Both columns are already on
the sheet; add idx_vs / idx_sync_id lookups and the matching keys
to the tx dict so the Python /api/* wire shape matches Go's
RawTransaction.

Update /api/* test fixtures to include vs/sync_id keys for realism.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_app.py

@@ -129,6 +129,7 @@ class TestWebApp(unittest.TestCase):
             'date': '2026-01-01', 'amount': 750, 'person': 'Test Member',
             'purpose': '2026-01', 'message': 'test payment',
             'sender': 'External Bank User', 'inferred_amount': 750,
+            'vs': '', 'sync_id': 'abc123',
         }]
         response = self.client.get('/api/adults')
         self.assertEqual(response.status_code, 200)
@@ -155,6 +156,7 @@ class TestWebApp(unittest.TestCase):
         mock_fetch_sheet.return_value = [{
             'date': '2026-01-15', 'amount': 500, 'person': 'Junior One',
             'purpose': '2026-01', 'message': '', 'sender': 'Parent', 'inferred_amount': 500,
+            'vs': '', 'sync_id': 'def456',
         }]
         response = self.client.get('/api/juniors')
         self.assertEqual(response.status_code, 200)
@@ -172,6 +174,7 @@ class TestWebApp(unittest.TestCase):
         mock_fetch_sheet.return_value = [{
             'date': '2026-01-01', 'amount': 750, 'person': 'Test Member',
             'purpose': '2026-01', 'message': 'test', 'sender': 'Someone',
+            'vs': '', 'sync_id': 'ghi789',
         }]
         response = self.client.get('/api/payments')
         self.assertEqual(response.status_code, 200)