added database migrations

migrations/20250903221313_overtime.down.sql

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+-- reverse: modify "wochen_report" table
+ALTER TABLE "wochen_report" DROP COLUMN "arbeitszeit", ALTER COLUMN "ueberstunden" TYPE smallint;
+-- reverse: modify "s_personal_daten" table
+ALTER TABLE "s_personal_daten" DROP COLUMN "arbeitszeit_per_woche";
+-- reverse: modify "s_abwesenheit_typen" table
+ALTER TABLE "s_abwesenheit_typen" DROP COLUMN "arbeitszeit_equivalent";