Refactor. Add data layer

BudgetApp.Data/Interfaces/ISalaryRepository.cs

@@ -0,0 +1,12 @@
+using BudgetApp.Data.Entities;
+
+namespace BudgetApp.Data.Interfaces;
+
+public interface ISalaryRepository
+{
+    Task<List<Salary>> GetSalariesAsync(string userId);
+    Task CreateSalaryAsync(Salary salary, DateTime windowEnd);
+    Task DeleteSalaryAsync(int id, string userId);
+    Task MaterializeSalaryTransactionsAsync(string userId, DateTime windowEnd);
+    Task<DateTime?> GetNextPaydateAsync(int salaryId);
+}