feat(core): add balance model

src/FiscalOS.Core/Accounts/Account.cs

@@ -8,6 +8,9 @@ public sealed class Account : Entity
   public string Name { get; init; } = string.Empty;
   public AccountMetadata? Metadata { get; init; }
 
+  private readonly List<Balance> _balances = [];
+  public IEnumerable<Balance> Balances => _balances;
+
   public static Account From(Guid institutionId, string name, AccountMetadata accountMetadata)
   {
     return new()
@@ -17,4 +20,11 @@ public sealed class Account : Entity
       Metadata = accountMetadata,
     };
   }
+
+  public void AddBalance(Balance balance)
+  {
+    ArgumentNullException.ThrowIfNull(balance);
+
+    _balances.Add(balance);
+  }
 }

src/FiscalOS.Core/Accounts/Balance.cs

@@ -0,0 +1,21 @@
+namespace FiscalOS.Core.Accounts;
+
+public sealed class Balance : Entity
+{
+  public Guid AccountId { get; init; }
+  public decimal Current { get; init; }
+  public decimal Available { get; init; }
+
+  private Balance()
+  {
+  }
+
+  public static Balance From(decimal current, decimal available)
+  {
+    return new()
+    {
+      Current = current,
+      Available = available,
+    };
+  }
+}