Add domain model with entities and enums

- Account, AccountYear, Entry, TransferLink entities
- EntryType enum (Income/Expense)
- Soft-delete support via IsDeleted flag

src/Duempelkas.Domain/Duempelkas.Domain.csproj

@@ -0,0 +1,7 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+</Project>

src/Duempelkas.Domain/Entities/Account.cs

@@ -0,0 +1,11 @@
+namespace Duempelkas.Domain.Entities;
+
+public class Account
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public decimal CarryoverBalance { get; set; }
+    public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
+
+    public ICollection<Entry> Entries { get; set; } = new List<Entry>();
+}

src/Duempelkas.Domain/Entities/AccountYear.cs

@@ -0,0 +1,13 @@
+namespace Duempelkas.Domain.Entities;
+
+public class AccountYear
+{
+    public int Id { get; set; }
+    public int AccountId { get; set; }
+    public int Year { get; set; }
+    public decimal OpeningBalance { get; set; }
+    public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
+
+    public Account Account { get; set; } = null!;
+    public ICollection<Entry> Entries { get; set; } = new List<Entry>();
+}

src/Duempelkas.Domain/Entities/Entry.cs

@@ -0,0 +1,20 @@
+using Duempelkas.Domain.Enums;
+
+namespace Duempelkas.Domain.Entities;
+
+public class Entry
+{
+    public int Id { get; set; }
+    public int AccountId { get; set; }
+    public string DisplayId { get; set; } = string.Empty;
+    public EntryType Type { get; set; }
+    public DateTime Date { get; set; }
+    public string Title { get; set; } = string.Empty;
+    public decimal Amount { get; set; }
+    public bool IsDeleted { get; set; }
+    public int? TransferLinkId { get; set; }
+    public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
+
+    public Account Account { get; set; } = null!;
+    public TransferLink? TransferLink { get; set; }
+}

src/Duempelkas.Domain/Entities/TransferLink.cs

@@ -0,0 +1,13 @@
+namespace Duempelkas.Domain.Entities;
+
+public class TransferLink
+{
+    public int Id { get; set; }
+    public int SourceEntryId { get; set; }
+    public int TargetEntryId { get; set; }
+    public string? Note { get; set; }
+    public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
+
+    public Entry SourceEntry { get; set; } = null!;
+    public Entry TargetEntry { get; set; } = null!;
+}

src/Duempelkas.Domain/Enums/EntryType.cs

@@ -0,0 +1,7 @@
+namespace Duempelkas.Domain.Enums;
+
+public enum EntryType
+{
+    Income = 0,
+    Expense = 1
+}