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.go

@@ -3,6 +3,7 @@ package matching
 import (
 	"fmt"
 	"fuj-management/go/internal/domain/czech"
+	"strings"
 	"time"
 )
 
@@ -17,12 +18,22 @@ type Transaction struct {
 }
 
 // InferredDetails is the result of InferTransactionDetails.
+// Months holds "YYYY-MM" keys, or exactly one "other:…" purpose when the
+// payment is not a monthly fee (see IsOtherPurpose).
 type InferredDetails struct {
 	Members    []Match
 	Months     []string
 	SearchText string
 }
 
+// IsOtherPurpose reports whether s marks a payment that is not a monthly fee:
+// it starts with "other:" (case-insensitive, leading whitespace ignored).
+// Used for both the Purpose column and the bank message.
+// Ports legacy/scripts/match_payments.py is_other_purpose.
+func IsOtherPurpose(s string) bool {
+	return strings.HasPrefix(strings.ToLower(strings.TrimSpace(s)), "other:")
+}
+
 // InferTransactionDetails infers which member(s) and month(s) a transaction belongs to.
 //
 // Search text for member matching: sender + message + user_id.
@@ -30,6 +41,10 @@ type InferredDetails struct {
 // Fallback 1: if no members found, retry match on sender alone.
 // Fallback 2: if no months found, derive from tx.Date (Sheets serial or YYYY-MM-DD).
 //
+// A message that starts with "other:" is not a monthly fee: no month is
+// inferred (neither from the text nor from the date) and Months holds the
+// trimmed message as the single purpose.
+//
 // defaultYear seeds czech.ParseMonthReferences (Python defaulted to the current year;
 // callers should pass time.Now().Year() or a fixed year for deterministic tests).
 //
@@ -38,12 +53,20 @@ func InferTransactionDetails(tx Transaction, memberNames []string, defaultYear i
 	searchText := fmt.Sprintf("%s %s %s", tx.Sender, tx.Message, tx.UserID)
 
 	members := MatchMembers(searchText, memberNames)
-	months := czech.ParseMonthReferences(tx.Message+" "+tx.UserID, defaultYear)
-
 	if len(members) == 0 {
 		members = MatchMembers(tx.Sender, memberNames)
 	}
 
+	if IsOtherPurpose(tx.Message) {
+		return InferredDetails{
+			Members:    members,
+			Months:     []string{strings.TrimSpace(tx.Message)},
+			SearchText: searchText,
+		}
+	}
+
+	months := czech.ParseMonthReferences(tx.Message+" "+tx.UserID, defaultYear)
+
 	if len(months) == 0 && tx.Date != nil && tx.Date != "" {
 		if ym := inferMonthFromDate(tx.Date); ym != "" {
 			months = []string{ym}

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()
 

go/internal/domain/reconcile/reconcile.go

@@ -203,7 +203,7 @@ func Reconcile(
 		personStr := strings.TrimSpace(tx.Person)
 		purposeStr := strings.TrimSpace(tx.Purpose)
 		personStr = questionMarkRe.ReplaceAllString(personStr, "")
-		isOther := strings.HasPrefix(strings.ToLower(purposeStr), "other:")
+		isOther := matching.IsOtherPurpose(purposeStr)
 
 		var matchedMembers []matching.Match
 		var matchedMonths []string
@@ -249,6 +249,11 @@ func Reconcile(
 			matchedMembers = inferred.Members
 			matchedMonths = inferred.Months
 			amount = tx.Amount
+			// A message starting with "other:" is inferred as a non-fee payment.
+			if len(matchedMonths) == 1 && matching.IsOtherPurpose(matchedMonths[0]) {
+				isOther = true
+				purposeStr = matchedMonths[0]
+			}
 		}
 
 		if len(matchedMembers) == 0 || len(matchedMonths) == 0 {

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}}}}

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": {