refactor: rename namespace to FiscalFlow and upgrade to .NET 10

- Rename InvoiceMaster.* to FiscalFlow.* namespace
- Upgrade from .NET 8 to .NET 10
- Update all NuGet packages to latest versions
- Update C# language version to 14.0

backend/src/FiscalFlow.Core/Entities/AccountingConnection.cs

@@ -0,0 +1,80 @@
+namespace FiscalFlow.Core.Entities;
+
+public class AccountingConnection : BaseEntity
+{
+    public Guid UserId { get; private set; }
+    public User User { get; private set; } = null!;
+    
+    public string Provider { get; private set; } = string.Empty;
+    
+    public string AccessTokenEncrypted { get; private set; } = string.Empty;
+    public string RefreshTokenEncrypted { get; private set; } = string.Empty;
+    public DateTime ExpiresAt { get; private set; }
+    public string? Scope { get; private set; }
+    
+    public string? CompanyName { get; private set; }
+    public string? CompanyOrgNumber { get; private set; }
+    
+    public string DefaultVoucherSeries { get; private set; } = "A";
+    public int DefaultAccountCode { get; private set; } = 5460;
+    public bool AutoAttachPdf { get; private set; } = true;
+    public bool AutoCreateSupplier { get; private set; } = false;
+    
+    public bool IsActive { get; private set; } = true;
+    public DateTime? LastSyncAt { get; private set; }
+
+    private readonly List<Invoice> _invoices = [];
+    public IReadOnlyCollection<Invoice> Invoices => _invoices.AsReadOnly();
+
+    public static AccountingConnection Create(
+        Guid userId,
+        string provider,
+        string accessTokenEncrypted,
+        string refreshTokenEncrypted,
+        DateTime expiresAt,
+        string? scope = null,
+        string? companyName = null,
+        string? companyOrgNumber = null)
+    {
+        return new AccountingConnection
+        {
+            UserId = userId,
+            Provider = provider,
+            AccessTokenEncrypted = accessTokenEncrypted,
+            RefreshTokenEncrypted = refreshTokenEncrypted,
+            ExpiresAt = expiresAt,
+            Scope = scope,
+            CompanyName = companyName,
+            CompanyOrgNumber = companyOrgNumber
+        };
+    }
+
+    public void UpdateTokens(string accessTokenEncrypted, string refreshTokenEncrypted, DateTime expiresAt)
+    {
+        AccessTokenEncrypted = accessTokenEncrypted;
+        RefreshTokenEncrypted = refreshTokenEncrypted;
+        ExpiresAt = expiresAt;
+    }
+
+    public void UpdateSettings(
+        string? defaultVoucherSeries = null,
+        int? defaultAccountCode = null,
+        bool? autoAttachPdf = null,
+        bool? autoCreateSupplier = null)
+    {
+        if (defaultVoucherSeries != null) DefaultVoucherSeries = defaultVoucherSeries;
+        if (defaultAccountCode.HasValue) DefaultAccountCode = defaultAccountCode.Value;
+        if (autoAttachPdf.HasValue) AutoAttachPdf = autoAttachPdf.Value;
+        if (autoCreateSupplier.HasValue) AutoCreateSupplier = autoCreateSupplier.Value;
+    }
+
+    public void RecordSync()
+    {
+        LastSyncAt = DateTime.UtcNow;
+    }
+
+    public void Deactivate()
+    {
+        IsActive = false;
+    }
+}