Add project files.

BudgetApp/Data/Transaction.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace BudgetApp.Data;
+
+public enum TransactionType
+{
+    Income,
+    Expense
+}
+
+public enum RecurrenceType
+{
+    None,
+    Daily,
+    Weekly,
+    Monthly,
+    Yearly
+}
+
+public class Transaction
+{
+    public int Id { get; set; }
+
+    [Required, MaxLength(200)]
+    public string Description { get; set; } = string.Empty;
+
+    public decimal Amount { get; set; }
+
+    public DateTime Date { get; set; } = DateTime.Today;
+
+    public TransactionType Type { get; set; } = TransactionType.Expense;
+
+    public RecurrenceType RecurrenceType { get; set; } = RecurrenceType.None;
+
+    public DateTime? RecurrenceEndDate { get; set; }
+
+    public int? ParentTransactionId { get; set; }
+    public Transaction? ParentTransaction { get; set; }
+
+    public int AccountId { get; set; }
+    public Account Account { get; set; } = default!;
+
+    public int? CategoryId { get; set; }
+    public Category? Category { get; set; }
+}