DataSourceAdapter-Pattern + SiteExportService schlanker + Page-Services Scoped

- IDataSourceAdapter mit 3 Implementierungen (HANA, SAP_GATEWAY, MANUAL_EXCEL)
  und DataSourceAdapterResolver ersetzen das if/else auf ConnectionKind.
- SiteExportService von 338 auf 187 Zeilen reduziert: Pipeline
  Resolve -> Fetch -> Transform -> Excel -> Central -> SharePoint.
- Page-Services auf Scoped (per Blazor-Circuit); Orchestrator bleibt Singleton
  fuer geteilten Export-Status.

TrafagSalesExporter/Services/DataSources/DataSourceCredentials.cs

@@ -0,0 +1,24 @@
+using TrafagSalesExporter.Models;
+
+namespace TrafagSalesExporter.Services.DataSources;
+
+internal static class DataSourceCredentials
+{
+    public static (string Username, string Password) Resolve(Site site, SourceSystemDefinition sourceDefinition)
+        => (FirstNonEmpty(site.UsernameOverride, sourceDefinition.CentralUsername),
+            FirstNonEmpty(site.PasswordOverride, sourceDefinition.CentralPassword));
+
+    public static string ResolveSapServiceUrl(Site site, SourceSystemDefinition sourceDefinition)
+        => FirstNonEmpty(site.SapServiceUrl, sourceDefinition.CentralServiceUrl);
+
+    public static string FirstNonEmpty(params string[] values)
+    {
+        foreach (var value in values)
+        {
+            if (!string.IsNullOrWhiteSpace(value))
+                return value.Trim();
+        }
+
+        return string.Empty;
+    }
+}