fix(go): pass raw value to FormatDate so numeric serial-day dates format

The transaction-row parser in services/membership/sources.go used a
helper (`getVal`) that did `fmt.Sprint(row[i])` before passing to
`matching.FormatDate`.  The Sheets API returns date-formatted cells
as `float64` (Sheets serial-day numbers); pre-stringifying defeated
`FormatDate`'s `case float64:` dispatch, so values like 46147 leaked
through unchanged as the string "46147" instead of being converted
to "2026-05-05".

Surfaced by `make parity` (M5.4) — every `transactions[].date` on
/api/adults and /api/juniors differed between Python and Go.  Python
side passes the raw value through directly (`isinstance(val, (int,
float))` in scripts/match_payments.py format_date), so it was always
correct.

Added a `getRaw` helper that returns row[i] without stringifying;
only the date column needs it.  Extended TestLoadTransactions with
a numeric-serial-day row to lock in the regression.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 2026-05-07 23:17 CEST — fix(go): pass raw value to FormatDate so numeric serial-day dates format
+
+- `go/internal/services/membership/sources.go`: transaction-row parser now passes `row[idxDate]` directly to `matching.FormatDate` (via a new `getRaw` helper) instead of stringifying first via `getVal`. The Sheets API returns numeric serial-day values as `float64` for date-formatted cells; pre-stringifying them defeated `FormatDate`'s `case float64:` dispatch, causing all numeric dates to leak through as `"46147"` style strings instead of `"2026-05-05"`.
+- Surfaced by `make parity` (M5.4): every `transactions[].date` field on `/api/adults` and `/api/juniors` differed between Python and Go.
+- `sources_test.go::TestLoadTransactions` extended with a numeric-serial-day row covering the regression.
+
 ## 2026-05-07 23:05 CEST — fix(go): default CacheDir to `tmp/go` to avoid Python collision
 
 - `go/internal/config/config.go`: `CacheDir` default changed from `tmp` to `tmp/go`. Override via `CACHE_DIR` env var still works.

go/internal/services/membership/sources.go

@@ -394,9 +394,19 @@ func parseTransactionRows(rows [][]any) ([]reconcile.Transaction, error) {
 		return fmt.Sprint(row[i])
 	}
 
+	// getRaw returns row[i] without stringifying — needed for FormatDate to
+	// dispatch on the underlying numeric type (Sheets returns serial-day
+	// numbers as float64). Stringifying first defeats that dispatch.
+	getRaw := func(row []any, i int) any {
+		if i < 0 || i >= len(row) {
+			return nil
+		}
+		return row[i]
+	}
+
 	var txns []reconcile.Transaction
 	for _, row := range rows[1:] {
-		dateStr := matching.FormatDate(getVal(row, idxDate))
+		dateStr := matching.FormatDate(getRaw(row, idxDate))
 		amountRaw := row[idxAmount]
 		if idxAmount < 0 || idxAmount >= len(row) {
 			amountRaw = ""

go/internal/services/membership/sources_test.go

@@ -114,12 +114,15 @@ func TestLoadJuniors(t *testing.T) {
 
 func TestLoadTransactions(t *testing.T) {
 	// Sheets fake keyed by "<spreadsheetID>/<range>" — use the real constant.
+	// Row 1 uses a pre-formatted date string; row 2 uses the numeric Sheets
+	// serial-day form (float64) — the API returns either depending on cell
+	// formatting, and FormatDate must handle both.
 	paymentsKey := config.PaymentsSheetID + "/A1:Z"
 	sh := &sheets.Fake{Values: map[string][][]any{
 		paymentsKey: {
 			{"Date", "Amount", "manual fix", "Person", "Purpose", "Inferred Amount", "Sender", "VS", "Message", "Bank ID", "Sync ID"},
 			{"2026-04-01", 700.0, "", "Alice", "2026-04", "", "Alice Bank", "", "fee", "", "abc"},
-			{"2026-05-01", 500.0, "", "", "", "", "Bob Bank", "", "platba", "", "def"},
+			{46147.0, 500.0, "", "", "", "", "Bob Bank", "", "platba", "", "def"}, // 46147 serial-day = 2026-05-05
 		},
 	}}
 	s := buildSources(t, &attendance.Fake{}, sh)
@@ -137,6 +140,12 @@ func TestLoadTransactions(t *testing.T) {
 	if txns[0].Amount != 700 {
 		t.Errorf("txn[0].Amount: want 700, got %v", txns[0].Amount)
 	}
+	if txns[0].Date != "2026-04-01" {
+		t.Errorf("txn[0].Date: want 2026-04-01, got %q", txns[0].Date)
+	}
+	if txns[1].Date != "2026-05-05" {
+		t.Errorf("txn[1].Date (numeric serial-day): want 2026-05-05, got %q", txns[1].Date)
+	}
 }
 
 func TestLoadExceptions(t *testing.T) {