Bilder auf WebP umstellen: ImageService + DB-Migration

- ImageService: PNG-Encoder durch WebP-Encoder ersetzt (Quality 80)
- Bilder werden jetzt immer zu WebP konvertiert (auch kleine)
- DB-Migration: Bestehende PNG/JPEG-Avatare und Session-Images
  automatisch zu WebP konvertiert beim Start
- Migration ist idempotent (überspringt bereits konvertierte Bilder)
- Kein Frontend-Code betroffen (Data-URLs sind format-agnostisch)

Services/DatabaseService.cs

@@ -1,10 +1,13 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
 using Dapper;
 using Gott.Models;
 using Microsoft.Data.Sqlite;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Formats.Webp;
 
 namespace Gott.Services;
 
@@ -258,6 +261,63 @@ public class DatabaseService
             CREATE INDEX IF NOT EXISTS idx_shared_messages_token ON shared_messages(share_token);
             CREATE INDEX IF NOT EXISTS idx_shared_messages_message ON shared_messages(message_id);
         ");
+
+        // Migration: Bestehende PNG/JPEG-Avatare/Images zu WebP konvertieren
+        MigrateImagesToWebP(conn);
+    }
+
+    private void MigrateImagesToWebP(SqliteConnection conn)
+    {
+        const int webPQuality = 80;
+
+        // Agent Avatare
+        var agentImages = conn.Query<(string AgentId, string Avatar)>(
+            "SELECT agent_id, avatar FROM agents WHERE avatar IS NOT NULL AND avatar != '' AND avatar NOT LIKE 'data:image/webp%'")
+            .ToList();
+
+        foreach (var (agentId, avatar) in agentImages)
+        {
+            var converted = ConvertDataUrlToWebP(avatar, webPQuality);
+            if (converted != null)
+                conn.Execute("UPDATE agents SET avatar = @converted WHERE agent_id = @agentId", new { converted, agentId });
+        }
+
+        // Session Images
+        var sessionImages = conn.Query<(string SessionId, string Image)>(
+            "SELECT session_id, image FROM sessions WHERE image IS NOT NULL AND image != '' AND image NOT LIKE 'data:image/webp%'")
+            .ToList();
+
+        foreach (var (sessionId, image) in sessionImages)
+        {
+            var converted = ConvertDataUrlToWebP(image, webPQuality);
+            if (converted != null)
+                conn.Execute("UPDATE sessions SET image = @converted WHERE session_id = @sessionId", new { converted, sessionId });
+        }
+
+        if (agentImages.Count > 0 || sessionImages.Count > 0)
+            Console.WriteLine($"[Migration] Converted {agentImages.Count} agent avatars and {sessionImages.Count} session images to WebP");
+    }
+
+    private static string ConvertDataUrlToWebP(string dataUrl, int quality)
+    {
+        try
+        {
+            var commaIndex = dataUrl.IndexOf(',');
+            if (commaIndex < 0) return null;
+
+            var base64 = dataUrl.Substring(commaIndex + 1);
+            var bytes = Convert.FromBase64String(base64);
+
+            using var image = Image.Load(bytes);
+            using var ms = new MemoryStream();
+            image.Save(ms, new WebpEncoder { Quality = quality });
+
+            return "data:image/webp;base64," + Convert.ToBase64String(ms.ToArray());
+        }
+        catch
+        {
+            return null; // Beschädigte Bilder überspringen
+        }
     }
 
     private void TryAddColumn(SqliteConnection conn, string table, string column, string type)