Add ItemType for determining which DB methods should be called based on the screen data. Begin implementing it.

lib/models/expense.dart

@@ -1,4 +1,6 @@
 // Local
+import 'package:flutter_expense_tracker/models/item_type.dart';
+
 import '/models/tracked_item.dart';
 import '/models/frequency.dart';
 
@@ -7,6 +9,7 @@ class Expense extends TrackedItem {
 
   Expense({
     super.id,
+    super.type = ItemType.expense,
     required super.name,
     required super.amount,
     required super.frequency,