feat: Thumbnail-Varianten (256px) für Agent-Avatare und Session-Images

- ImageService: ResizeDataUrlSmall() für 256px WebP-Thumbnails (Quality 70)
- Neue DB-Spalten: agents.avatar_small, sessions.image_small
- Migration generiert Thumbnails für bestehende Bilder automatisch
- Listen-Endpoints (/api/agents, /api/sessions) liefern kleine Variante
- Detail-Endpoints liefern weiterhin das volle 512px-Bild
- Frontend unverändert (Cards nutzen automatisch die kleineren Bilder)

Services/ImageService.cs

@@ -9,7 +9,9 @@ namespace Gott.Services;
 public class ImageService
 {
     private const int MaxSideLength = 512;
+    private const int SmallSideLength = 256;
     private const int WebPQuality = 80;
+    private const int WebPQualitySmall = 70;
 
     /// <summary>
     /// Nimmt eine Base64-Data-URL (z.B. "data:image/png;base64,..."), skaliert das Bild
@@ -17,6 +19,20 @@ public class ImageService
     /// Gibt null zurück wenn input null/leer ist.
     /// </summary>
     public string ResizeDataUrl(string dataUrl)
+    {
+        return ResizeToWebP(dataUrl, MaxSideLength, WebPQuality);
+    }
+
+    /// <summary>
+    /// Erzeugt eine kleine Thumbnail-Variante (max 256px, Quality 70) für Dashboard-Listen.
+    /// Gibt null zurück wenn input null/leer ist.
+    /// </summary>
+    public string ResizeDataUrlSmall(string dataUrl)
+    {
+        return ResizeToWebP(dataUrl, SmallSideLength, WebPQualitySmall);
+    }
+
+    private static string ResizeToWebP(string dataUrl, int maxSide, int quality)
     {
         if (string.IsNullOrEmpty(dataUrl)) return null;
 
@@ -30,18 +46,18 @@ public class ImageService
         using var image = Image.Load(bytes);
 
         // Skalieren wenn nötig
-        if (image.Width > MaxSideLength || image.Height > MaxSideLength)
+        if (image.Width > maxSide || image.Height > maxSide)
         {
             image.Mutate(x => x.Resize(new ResizeOptions
             {
-                Size = new Size(MaxSideLength, MaxSideLength),
+                Size = new Size(maxSide, maxSide),
                 Mode = ResizeMode.Max
             }));
         }
 
         // Immer als WebP encodieren
         using var ms = new MemoryStream();
-        image.Save(ms, new WebpEncoder { Quality = WebPQuality });
+        image.Save(ms, new WebpEncoder { Quality = quality });
         var webpBase64 = Convert.ToBase64String(ms.ToArray());
 
         return "data:image/webp;base64," + webpBase64;