feat(infer): messages starting with "other:" are not monthly fees

When the bank message starts with "other:" (case-insensitive),
inference does not look for a month, neither in the text nor from the
payment date. The trimmed message is written to Purpose, so reconcile
books the payment as an "other" transaction for the person instead of
a monthly fee. Messages containing "other:" elsewhere are unchanged.

- matching.IsOtherPurpose / is_other_purpose shared by inference and
  reconcile
- reconcile applies the rule to rows not yet processed by inference
- no person found: Purpose is still written, row shows as unmatched
- implemented in Go and legacy Python; four new parity fixtures

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

legacy/tests/test_match_payments.py

@@ -1,7 +1,9 @@
 import unittest
 from unittest.mock import MagicMock, patch
 
-from scripts.match_payments import canonical_member_key, fetch_sheet_data, reconcile
+from scripts.match_payments import (
+    canonical_member_key, fetch_sheet_data, infer_transaction_details, is_other_purpose, reconcile,
+)
 
 
 class TestCanonicalMemberKey(unittest.TestCase):
@@ -88,3 +90,44 @@ class TestFetchSheetDataVS(unittest.TestCase):
 
     def test_string_vs_is_kept(self):
         self.assertEqual(self._fetch("0308")[0]["vs"], "0308")
+
+
+class TestOtherPurposeInference(unittest.TestCase):
+    """A message starting with "other:" is not a monthly fee."""
+
+    MEMBERS = ["Jana Nováková", "Adam Novák"]
+
+    def test_is_other_purpose(self):
+        for text in ("other: tournament", "other:shirt", "  Other: Tournament", "OTHER:x"):
+            self.assertTrue(is_other_purpose(text), text)
+        for text in ("leden other: x", "other tournament", "another: x", "2026-01", "", None):
+            self.assertFalse(is_other_purpose(text), text)
+
+    def test_message_becomes_purpose_and_no_month_is_inferred(self):
+        tx = {"sender": "Jana Novakova", "message": " other: turnaj leden 2026 ", "date": "2026-04-15"}
+        result = infer_transaction_details(tx, self.MEMBERS)
+        self.assertEqual(result["members"], [("Jana Nováková", "auto")])
+        self.assertEqual(result["months"], ["other: turnaj leden 2026"])
+
+    def test_unknown_member_still_yields_purpose(self):
+        tx = {"sender": "Unknown Payer", "message": "Other: Dres", "date": "2026-04-15"}
+        result = infer_transaction_details(tx, self.MEMBERS)
+        self.assertEqual(result["members"], [])
+        self.assertEqual(result["months"], ["Other: Dres"])
+
+    def test_other_not_at_start_is_handled_the_old_way(self):
+        tx = {"sender": "Jana Novakova", "message": "leden other: dres", "date": "2026-04-15"}
+        result = infer_transaction_details(tx, self.MEMBERS)
+        self.assertEqual(result["months"], ["2026-01"])
+
+    def test_reconcile_fallback_puts_it_in_other_ledger(self):
+        members = [("Jana Nováková", "A", {"2026-01": (750, 3)})]
+        tx = {"date": "2026-01-10", "amount": 300, "person": "", "purpose": "",
+              "sender": "Jana Novakova", "message": "other: dres", "inferred_amount": ""}
+        result = reconcile(members, ["2026-01"], [tx], {})
+        member = result["members"]["Jana Nováková"]
+        self.assertEqual(member["months"]["2026-01"]["paid"], 0)
+        self.assertEqual(len(member["other_transactions"]), 1)
+        self.assertEqual(member["other_transactions"][0]["amount"], 300)
+        self.assertEqual(member["other_transactions"][0]["purpose"], "other: dres")
+        self.assertEqual(result["unmatched"], [])