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}