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/SessionEndpoint.cs

@@ -66,7 +66,7 @@ public static class SessionEndpoint
                 s.SessionId,
                 s.Name,
                 s.Description,
-                s.Image,
+                Image = s.ImageSmall ?? s.Image,
                 s.AgentId,
                 s.Workspace,
                 s.Context,
@@ -120,6 +120,7 @@ public static class SessionEndpoint
         var description = body.TryGetProperty("description", out var d) ? d.GetString() : null;
         var imageRaw = body.TryGetProperty("image", out var img) ? img.GetString() : null;
         var image = imageService.ResizeDataUrl(imageRaw);
+        var imageSmall = imageService.ResizeDataUrlSmall(imageRaw);
         var agentId = body.TryGetProperty("agentId", out var a) ? a.GetString() : null;
         var workspaceRaw = body.TryGetProperty("workspace", out var w) ? w.GetString() : null;
         // Relativen Workspace validieren (Path-Traversal verhindern)
@@ -142,7 +143,7 @@ public static class SessionEndpoint
         var finalizerAgentId = body.TryGetProperty("finalizerAgentId", out var fa) ? fa.GetString() : null;
         var preparerAgentId = body.TryGetProperty("preparerAgentId", out var pra) ? pra.GetString() : null;
 
-        var session = await db.CreateSession(userId, name, description, image, agentId, workspace, context, customCommands, avatarScale, avatarOffsetX, avatarOffsetY, reviewAgentId, maxReviewIterations, finalizerAgentId, preparerAgentId);
+        var session = await db.CreateSession(userId, name, description, image, imageSmall, agentId, workspace, context, customCommands, avatarScale, avatarOffsetX, avatarOffsetY, reviewAgentId, maxReviewIterations, finalizerAgentId, preparerAgentId);
         await ctx.Response.WriteAsJsonAsync(session);
     }