refactor: Backend deduplizieren – 13 Dateien, -528 Zeilen netto

ChatContext: 3 identische Context-Klassen (PreparerContext, ReviewContext,
FinalizerContext) zu einer SubAgentContext-Klasse zusammengefasst.

ChatService: 3x40 Zeilen Sub-Agent-Setup durch BuildSubAgentContext() ersetzt.

AnthropicService: 5 Hilfsmethoden extrahiert – ExtractWebSearchSources(),
TruncateToolResult(), AddToolsToPayload(), ParseServerToolUse(), TrackUsage().

WorkspaceEndpoint: 5 Session/Agent-Endpoint-Paare durch generische Core-Handler
mit basePath-Parameter ersetzt. Ungenutzte rmdir-Endpoints entfernt.

AgentEndpoint: Doppeltes Agent-DTO-Mapping durch MapAgentList() ersetzt.

SessionEndpoint: Doppelte Session/Agent-Auflösung durch ResolvePromptContext() ersetzt.

DatabaseService: Toten Code entfernt (SetSessionTelegramChatId,
ClearSessionTelegramChatId, GetSessions, GetUserSessions). ConvertDataUrlToWebP
und ResizeDataUrlToWebP zu einer Methode zusammengefasst.

TelegramEndpoint: Bot-DTO-Mapping durch MapBotResponse() dedupliziert.

KreaEndpoint + SkillEndpoint: Thumbnail-Generierung in ImageService.GenerateThumbnail()
extrahiert, SixLabors-Imports aus Endpoints entfernt.

CronjobEndpoint: 7x ctx.Items[UserId] durch ctx.GetUserId() ersetzt.

Services/DatabaseService.cs

@@ -338,7 +338,7 @@ public class DatabaseService
 
         foreach (var (agentId, avatar) in agentsWithoutThumb)
         {
-            var small = ResizeDataUrlToWebP(avatar, smallSize, smallQuality);
+            var small = ConvertDataUrlToWebP(avatar, smallQuality, maxSide: smallSize);
             if (small != null)
                 conn.Execute("UPDATE agents SET avatar_small = @small WHERE agent_id = @agentId", new { small, agentId });
         }
@@ -350,7 +350,7 @@ public class DatabaseService
 
         foreach (var (sessionId, image) in sessionsWithoutThumb)
         {
-            var small = ResizeDataUrlToWebP(image, smallSize, smallQuality);
+            var small = ConvertDataUrlToWebP(image, smallQuality, maxSide: smallSize);
             if (small != null)
                 conn.Execute("UPDATE sessions SET image_small = @small WHERE session_id = @sessionId", new { small, sessionId });
         }
@@ -359,7 +359,10 @@ public class DatabaseService
             Console.WriteLine($"[Migration] Generated {agentsWithoutThumb.Count} agent thumbnails and {sessionsWithoutThumb.Count} session thumbnails");
     }
 
-    private static string ResizeDataUrlToWebP(string dataUrl, int maxSide, int quality)
+    /// <summary>
+    /// Konvertiert ein DataUrl-Bild zu WebP. Optional Resize wenn maxSide > 0.
+    /// </summary>
+    private static string ConvertDataUrlToWebP(string dataUrl, int quality, int maxSide = 0)
     {
         try
         {
@@ -370,7 +373,7 @@ public class DatabaseService
             var bytes = Convert.FromBase64String(base64);
 
             using var image = Image.Load(bytes);
-            if (image.Width > maxSide || image.Height > maxSide)
+            if (maxSide > 0 && (image.Width > maxSide || image.Height > maxSide))
             {
                 image.Mutate(x => x.Resize(new ResizeOptions
                 {
@@ -390,28 +393,6 @@ public class DatabaseService
         }
     }
 
-    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)
     {
         try { conn.Execute($"ALTER TABLE {table} ADD COLUMN {column} {type}"); }
@@ -573,13 +554,6 @@ public class DatabaseService
         return session;
     }
 
-    public async Task<IEnumerable<SessionRecord>> GetSessions(string userId)
-    {
-        using var conn = CreateConnection();
-        return await conn.QueryAsync<SessionRecord>(
-            "SELECT * FROM sessions WHERE user_id = @userId AND is_deleted = 0 ORDER BY updated_at DESC", new { userId });
-    }
-
     public async Task<List<SessionWithStats>> GetSessionsWithStats(string userId)
     {
         using var conn = CreateConnection();
@@ -1303,23 +1277,6 @@ public class DatabaseService
             new { sessionId, chatId, now });
     }
 
-    public async Task SetSessionTelegramChatId(string sessionId, string chatId)
-    {
-        using var conn = CreateConnection();
-        var now = DateTime.UtcNow.ToString("o");
-        await conn.ExecuteAsync(
-            "UPDATE sessions SET telegram_chat_id = @chatId, updated_at = @now WHERE session_id = @sessionId",
-            new { sessionId, chatId, now });
-    }
-
-    public async Task ClearSessionTelegramChatId(string sessionId)
-    {
-        using var conn = CreateConnection();
-        await conn.ExecuteAsync(
-            "UPDATE sessions SET telegram_chat_id = NULL, updated_at = @now WHERE session_id = @sessionId",
-            new { sessionId, now = DateTime.UtcNow.ToString("o") });
-    }
-
     public async Task<IEnumerable<SessionRecord>> GetSessionsByTelegramBot(string botId)
     {
         using var conn = CreateConnection();
@@ -1345,14 +1302,6 @@ public class DatabaseService
             new { botId });
     }
 
-    public async Task<IEnumerable<SessionRecord>> GetUserSessions(string userId)
-    {
-        using var conn = CreateConnection();
-        return await conn.QueryAsync<SessionRecord>(
-            "SELECT * FROM sessions WHERE user_id = @userId AND is_deleted = 0 ORDER BY updated_at DESC",
-            new { userId });
-    }
-
     // ── Cronjobs ──
 
     public async Task<CronjobRecord> CreateCronjob(CronjobRecord cronjob)