Refactor SQL scripts for index creation in sick and vacation tables; implement checks to prevent duplicate index creation and streamline execution process. Update TimeEntryService to remove obsolete overtime calculation method for cleaner code.

backend/add-vacation-index.sql

@@ -1,9 +1,22 @@
 -- Füge Index für vacation Tabelle hinzu
 -- Optimiert die Abfrage: WHERE user_id = ? AND first_day >= ? ORDER BY first_day DESC
+-- Ausführen mit: mysql -u root -p timeclock < add-vacation-index.sql
 
-CREATE INDEX idx_vacation_user_first_day 
-ON vacation (user_id, first_day DESC);
+-- Prüfe zuerst ob der Index bereits existiert
+SELECT COUNT(*) INTO @index_exists 
+FROM information_schema.statistics 
+WHERE table_schema = DATABASE() 
+  AND table_name = 'vacation' 
+  AND index_name = 'idx_vacation_user_first_day';
 
--- Prüfe ob der Index erstellt wurde
-SHOW INDEX FROM vacation WHERE Key_name = 'idx_vacation_user_first_day';
+SET @sql = IF(@index_exists = 0,
+  'CREATE INDEX idx_vacation_user_first_day ON vacation (user_id, first_day DESC)',
+  'SELECT "Index idx_vacation_user_first_day existiert bereits" AS Info'
+);
+
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+SELECT 'Index idx_vacation_user_first_day erfolgreich erstellt oder bereits vorhanden.' AS Status;