feat(go): M5.2 — HTTP handlers for /api/adults, /api/juniors, /api/payments, /api/version

- Add web/api/handler.go: Handler struct wiring Sources+Config into ServeAdults,
  ServeJuniors, ServePayments, ServeVersion
- Add web/api/build_common.go: getMonthLabels, groupRawPaymentsByPerson, settledBalance,
  domain-to-wire converters, ensureSlice generic helper
- Add web/api/build_adults.go: buildAdultsResponse + buildAdultMemberRow mirroring
  scripts/views.py:build_adults_view_model
- Add web/api/build_juniors.go: buildJuniorsResponse + buildJuniorMemberRow mirroring
  scripts/views.py:build_juniors_view_model, including "?" sentinel and :NJ,MA breakdown
- Add web/api/build_payments.go: buildPaymentsResponse with Unmatched/Unknown bucket
- Extend reconcile.FeeData/MonthData with IsUnknown, JuniorAttendance, AdultAttendance
- Extend reconcile.Transaction with ManualFix, VS, BankID, SyncID for raw_payments wire field
- Export membership.AdultMergedMonths and JuniorMergedMonths
- Update sources.go to propagate new FeeData fields and parse extra transaction columns
- Wire sources+cfg into web.Run; register /api/* routes via Go 1.22 method+path patterns
- Fix pre-existing gofumpt formatting in fio_test.go and fio_table.go

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go/internal/web/api/build_payments.go

@@ -0,0 +1,44 @@
+package api
+
+import (
+	"fuj-management/go/internal/config"
+	"sort"
+	"strings"
+
+	domreconcile "fuj-management/go/internal/domain/reconcile"
+)
+
+// buildPaymentsResponse constructs the PaymentsResponse wire type.
+// Mirrors scripts/views.py:build_payments_view_model.
+func buildPaymentsResponse(
+	txns []domreconcile.Transaction,
+	memberNames []string,
+) PaymentsResponse {
+	grouped := groupRawPaymentsByPerson(txns, memberNames)
+
+	// Add unmatched/unknown bucket for transactions with no person set.
+	const unknownKey = "Unmatched / Unknown"
+	for _, tx := range txns {
+		if strings.TrimSpace(tx.Person) == "" {
+			grouped[unknownKey] = append(grouped[unknownKey], rawTxFromDomain(tx))
+		}
+	}
+	// Sort the unknown bucket newest-first (others are sorted in groupRawPaymentsByPerson).
+	if rows, ok := grouped[unknownKey]; ok {
+		sort.Slice(rows, func(i, j int) bool { return rows[i].Date > rows[j].Date })
+		grouped[unknownKey] = rows
+	}
+
+	sortedPeople := make([]string, 0, len(grouped))
+	for p := range grouped {
+		sortedPeople = append(sortedPeople, p)
+	}
+	sort.Strings(sortedPeople)
+
+	return PaymentsResponse{
+		GroupedPayments: grouped,
+		SortedPeople:    sortedPeople,
+		AttendanceURL:   "https://docs.google.com/spreadsheets/d/" + config.AttendanceSheetID + "/edit",
+		PaymentsURL:     "https://docs.google.com/spreadsheets/d/" + config.PaymentsSheetID + "/edit",
+	}
+}