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)

Endpoints/AgentEndpoint.cs

@@ -42,7 +42,7 @@ public static class AgentEndpoint
             {
                 a.AgentId,
                 a.Name,
-                a.Avatar,
+                Avatar = a.AvatarSmall ?? a.Avatar,
                 a.Description,
                 a.Model,
                 a.ToolChoice,
@@ -96,6 +96,7 @@ public static class AgentEndpoint
             UserId = userId,
             Name = body.TryGetProperty("name", out var n) ? n.GetString() ?? "Neuer Agent" : "Neuer Agent",
             Avatar = imageService.ResizeDataUrl(avatarRaw),
+            AvatarSmall = imageService.ResizeDataUrlSmall(avatarRaw),
             Description = body.TryGetProperty("description", out var d) ? d.GetString() : null,
             Model = body.TryGetProperty("model", out var m) ? m.GetString() ?? "sonnet" : "sonnet",
             ToolChoice = body.TryGetProperty("toolChoice", out var tc) ? tc.GetString() ?? "any" : "any",
@@ -131,7 +132,11 @@ public static class AgentEndpoint
         var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
 
         if (body.TryGetProperty("name", out var n)) agent.Name = n.GetString();
-        if (body.TryGetProperty("avatar", out var av)) agent.Avatar = imageService.ResizeDataUrl(av.GetString());
+        if (body.TryGetProperty("avatar", out var av))
+        {
+            agent.Avatar = imageService.ResizeDataUrl(av.GetString());
+            agent.AvatarSmall = imageService.ResizeDataUrlSmall(av.GetString());
+        }
         if (body.TryGetProperty("description", out var d)) agent.Description = d.GetString();
         if (body.TryGetProperty("model", out var m)) agent.Model = m.GetString();
         if (body.TryGetProperty("toolChoice", out var tc)) agent.ToolChoice = tc.GetString();