Refactor EntryService to allow shared DisplayId for transfers; update related tests and migration files

tests/Duempelkas.Tests/YearlyStatementCalculationTests.cs

@@ -13,11 +13,12 @@ public class BalanceCalculationTests : IDisposable
     private readonly FinanceDbContext _db;
     private readonly BalanceQueryService _balanceQueryService;
     private readonly EntryService _entryService;
+    private readonly string _connectionString = $"Data Source=duempelkas-balance-tests-{Guid.NewGuid():N};Mode=Memory;Cache=Shared";
 
     public BalanceCalculationTests()
     {
         var options = new DbContextOptionsBuilder<FinanceDbContext>()
-            .UseSqlite("Data Source=duempelkas-balance-tests;Mode=Memory;Cache=Shared")
+            .UseSqlite(_connectionString)
             .Options;
 
         _db = new FinanceDbContext(options);