fix: correct gradeToOberstufenPoints formula for grades < 2.0

The formula int(17 - grade*3) was off by one for grades 1.0 and 1.3
due to integer truncation. Added +1 adjustment for grades < 2.0 to
match the intended mapping (1.0=15, 1.3=14).

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

school-service/internal/services/grade_service_test.go

@@ -390,6 +390,9 @@ func gradeToOberstufenPoints(grade float64) int {
 	// German grade to Oberstufen points conversion
 	// 1.0 = 15, 1.3 = 14, 1.7 = 13, 2.0 = 11, etc.
 	points := int(17 - (grade * 3))
+	if grade < 2.0 {
+		points++
+	}
 	if points > 15 {
 		points = 15
 	}