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/matching/infer_test.go

@@ -32,6 +32,74 @@ import (
 
 var inferMembers = []string{"Tomáš Němeček (Tov)", "Jana Nováková"}
 
+func TestIsOtherPurpose(t *testing.T) {
+	t.Parallel()
+	cases := map[string]bool{
+		"other: tournament":   true,
+		"other:shirt":         true,
+		"  Other: Tournament": true,
+		"OTHER:x":             true,
+		"leden other: x":      false,
+		"other tournament":    false,
+		"another: x":          false,
+		"2026-01":             false,
+		"":                    false,
+	}
+	for in, want := range cases {
+		if got := IsOtherPurpose(in); got != want {
+			t.Errorf("IsOtherPurpose(%q) = %v, want %v", in, got, want)
+		}
+	}
+}
+
+func TestInferTransactionDetails_OtherMessage(t *testing.T) {
+	t.Parallel()
+	jana := []Match{{Name: "Jana Nováková", Confidence: ConfidenceAuto}}
+	cases := []struct {
+		name        string
+		tx          Transaction
+		wantMembers []Match
+		wantMonths  []string
+	}{
+		{
+			// Month words and the date must both be ignored.
+			name:        "other prefix: message becomes the purpose, no month",
+			tx:          Transaction{Sender: "Jana Novakova", Message: " other: turnaj leden 2026 ", Date: "2026-04-15"},
+			wantMembers: jana,
+			wantMonths:  []string{"other: turnaj leden 2026"},
+		},
+		{
+			name:        "other prefix is case-insensitive and keeps the original text",
+			tx:          Transaction{Sender: "Jana Novakova", Message: "Other: Dres", Date: "2026-04-15"},
+			wantMembers: jana,
+			wantMonths:  []string{"Other: Dres"},
+		},
+		{
+			name:        "other prefix with no known member still yields the purpose",
+			tx:          Transaction{Sender: "Unknown Payer", Message: "other: dres", Date: "2026-04-15"},
+			wantMembers: nil,
+			wantMonths:  []string{"other: dres"},
+		},
+		{
+			name:        "other: not at the start is handled the old way",
+			tx:          Transaction{Sender: "Jana Novakova", Message: "leden other: dres", Date: "2026-04-15"},
+			wantMembers: jana,
+			wantMonths:  []string{"2026-01"},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := InferTransactionDetails(tc.tx, inferMembers, 2026)
+			if len(got.Members) != len(tc.wantMembers) || (len(got.Members) > 0 && !reflect.DeepEqual(got.Members, tc.wantMembers)) {
+				t.Errorf("Members = %v, want %v", got.Members, tc.wantMembers)
+			}
+			if !reflect.DeepEqual(got.Months, tc.wantMonths) {
+				t.Errorf("Months = %v, want %v", got.Months, tc.wantMonths)
+			}
+		})
+	}
+}
+
 func TestInferTransactionDetails(t *testing.T) {
 	t.Parallel()