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/DataSourceAdapterResolver.cs

@@ -0,0 +1,27 @@
+using TrafagSalesExporter.Models;
+
+namespace TrafagSalesExporter.Services.DataSources;
+
+public sealed class DataSourceAdapterResolver : IDataSourceAdapterResolver
+{
+    private readonly Dictionary<string, IDataSourceAdapter> _adapters;
+
+    public DataSourceAdapterResolver(IEnumerable<IDataSourceAdapter> adapters)
+    {
+        _adapters = adapters.ToDictionary(
+            a => a.ConnectionKind,
+            StringComparer.OrdinalIgnoreCase);
+    }
+
+    public IDataSourceAdapter Resolve(string connectionKind)
+    {
+        if (string.IsNullOrWhiteSpace(connectionKind))
+            connectionKind = SourceSystemConnectionKinds.Hana;
+
+        if (_adapters.TryGetValue(connectionKind, out var adapter))
+            return adapter;
+
+        throw new InvalidOperationException(
+            $"Kein DataSourceAdapter fuer ConnectionKind '{connectionKind}' registriert.");
+    }
+}