fix(go): show "Expected: ?" in junior tooltip for unknown fees

A junior month with one session has an unknown expected fee. When such
a month had a payment, the Go tooltip printed "Expected: 0" where
Python prints "Expected: ?", which made make parity fail on
/api/juniors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/web/api/build_juniors.go

@@ -242,7 +242,12 @@ func buildJuniorMemberRow(
 
 		tooltip := ""
 		if (!md.IsUnknown && md.Expected > 0) || paid > 0 {
-			tooltip = fmt.Sprintf("Received: %d, Expected: %d", paid, md.Expected)
+			// Unknown expected (one session, manual review) shows as "?", as in Python.
+			expectedText := strconv.Itoa(md.Expected)
+			if md.IsUnknown {
+				expectedText = "?"
+			}
+			tooltip = fmt.Sprintf("Received: %d, Expected: %s", paid, expectedText)
 		}
 
 		row.Months = append(row.Months, MonthCell{