fix: changed constraint for exercises.load column: "absolute" => "external"

internal/database/migrations/000005_rename_workout_load.down.sql

@@ -0,0 +1,10 @@
+BEGIN;
+
+ALTER TABLE exercises DROP CONSTRAINT exercises_load_check;
+
+UPDATE exercises SET load = 'absolute' WHERE load = 'external';
+
+ALTER TABLE exercises ADD CONSTRAINT exercises_load_check
+    CHECK (load IN ('bodyweight', 'absolute'));
+
+COMMIT;