fix(go): accept single-digit day/month in attendance date headers

parseDates was using "02.01.2006" / "01/02/2006" which require
zero-padded fields. The Czech attendance sheet headers contain dates
like "1.6.2026", "23.3.2026", "6.4.2026" — Go silently dropped those
columns while Python's strptime accepted them. Effect was a missing
2026-06 month on /api/juniors plus undercounted attendance in any month
with single-digit columns; surfaced via make parity.

Use the unpadded reference forms "2.1.2006" / "1/2/2006" instead — Go's
time.Parse accepts both padded and unpadded inputs against them.

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

go/internal/services/membership/sources.go

@@ -142,7 +142,13 @@ func parseDates(header []string) []struct {
 		}
 		var dt time.Time
 		var err error
-		for _, fmt_ := range []string{"02.01.2006", "01/02/2006"} {
+		// Use the unpadded reference forms ("2.1" and "1/2"): Go's time.Parse
+		// accepts both single-digit and zero-padded inputs against them, so
+		// "1.6.2026", "01.06.2026", "23.3.2026" all parse. Czech sheet authors
+		// drop the leading zero on dates ≤ 9 — Python's strptime is lenient
+		// the same way; the previous "02.01.2006" form silently dropped those
+		// columns and undercounted attendance.
+		for _, fmt_ := range []string{"2.1.2006", "1/2/2006"} {
 			dt, err = time.Parse(fmt_, raw)
 			if err == nil {
 				break

go/internal/services/membership/sources_test.go

@@ -174,6 +174,28 @@ func TestLoadExceptions(t *testing.T) {
 	}
 }
 
+// TestParseDates_SingleDigitDayMonth covers the regression where Go's strict
+// "02.01.2006" format dropped header cells written without leading zeros
+// (e.g. "1.6.2026", "23.3.2026"), causing attendance undercounts and missing
+// months on the /api/juniors response. Czech sheet authors drop the zero
+// pad freely; Python's strptime tolerates it, so the parsers must match.
+func TestParseDates_SingleDigitDayMonth(t *testing.T) {
+	// Czech form ("DD.MM.YYYY", with leading zeros optional) is the primary
+	// path. The "M/D/YYYY" fallback mirrors Python's %m/%d/%Y secondary
+	// strptime branch — month-first, day-second.
+	header := []string{"Jméno", "Tier", "", "01.06.2026", "1.6.2026", "23.3.2026", "6.4.2026", "01/02/2026", "1/2/2026"}
+	got := parseDates(header)
+	want := []string{"2026-06", "2026-06", "2026-03", "2026-04", "2026-01", "2026-01"}
+	if len(got) != len(want) {
+		t.Fatalf("parseDates: got %d entries, want %d (%v)", len(got), len(want), got)
+	}
+	for i, e := range got {
+		if e.month != want[i] {
+			t.Errorf("parseDates[%d].month = %q, want %q (raw=%q)", i, e.month, want[i], header[e.col])
+		}
+	}
+}
+
 // TTL smoke test: second call within TTL must not call fetch again.
 func TestLoadAdults_CacheHit(t *testing.T) {
 	dir := t.TempDir()