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/services/banksync/infer_test.go

@@ -53,6 +53,42 @@ func TestInferPayments_BasicMatch(t *testing.T) {
 	}
 }
 
+func TestInferPayments_OtherMessage(t *testing.T) {
+	sh := &sheets.Fake{Values: map[string][][]any{
+		"SHEETID/A1:Z": {
+			paymentsHeader,
+			// Known member: Person filled, message copied to Purpose, no month.
+			{"2026-04-10", 300.0, "", "", "", "", "Jana Novakova", "", "other: dres", "", ""},
+			// Unknown sender: Purpose is still written, Person stays empty.
+			{"2026-04-11", 500.0, "", "", "", "", "Unknown Payer", "", "Other: turnaj", "", ""},
+		},
+	}}
+	att := &fakeAttendance{
+		adults: []reconcile.Member{{Name: "Jana Novakova", Tier: "A"}},
+	}
+
+	n, err := InferPayments(context.Background(), "SHEETID", sh, att, InferOpts{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 2 {
+		t.Fatalf("want 2 rows updated, got %d", n)
+	}
+	updates := sh.BatchUpdated[0].Updates
+	want := [][]any{
+		{"Jana Novakova", "other: dres", "300"},
+		{"", "Other: turnaj", "500"},
+	}
+	for i, w := range want {
+		got := updates[i].Values[0]
+		for j := range w {
+			if got[j] != w[j] {
+				t.Errorf("row %d col %d: want %q, got %q", i, j, w[j], got[j])
+			}
+		}
+	}
+}
+
 func TestInferPayments_SkipRule_ManualFix(t *testing.T) {
 	sh := &sheets.Fake{Values: map[string][][]any{
 		"SHEETID/A1:Z": {