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>

go/internal/domain/reconcile/reconcile_test.go

@@ -257,6 +257,33 @@ func TestReconcileQuestionMarkMarkerStripped(t *testing.T) {
 }
 
 // [Go] Purpose "other:shirt" puts payment in OtherTransactions, not in month ledger.
+// A row not yet processed by inference (empty Person/Purpose) whose message
+// starts with "other:" must land in the other ledger, not in a month.
+func TestReconcileOtherMessageFallback(t *testing.T) {
+	t.Parallel()
+	members := []Member{{Name: "Alice", Tier: "A", Fees: map[string]FeeData{"2026-01": {Expected: 750, Attendance: 3}}}}
+	txs := []Transaction{{
+		Date: "2026-01-10", Amount: 300,
+		Sender: "Alice", Message: "other: shirt",
+	}}
+
+	result := Reconcile(members, []string{"2026-01"}, txs, nil, 2026)
+
+	alice := result.Members["Alice"]
+	if paid := alice.Months["2026-01"].Paid; paid != 0 {
+		t.Errorf("2026-01 paid = %v, want 0", paid)
+	}
+	if len(alice.OtherTransactions) != 1 {
+		t.Fatalf("other transactions = %d, want 1", len(alice.OtherTransactions))
+	}
+	if got := alice.OtherTransactions[0]; got.Amount != 300 || got.Purpose != "other: shirt" {
+		t.Errorf("other entry = %+v", got)
+	}
+	if len(result.Unmatched) != 0 {
+		t.Errorf("unmatched = %d, want 0", len(result.Unmatched))
+	}
+}
+
 func TestReconcileOtherPurpose(t *testing.T) {
 	t.Parallel()
 	members := []Member{{Name: "Alice", Tier: "A", Fees: map[string]FeeData{"2026-01": {Expected: 750, Attendance: 3}}}}