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.

Endpoints/CronjobEndpoint.cs

@@ -2,6 +2,7 @@ using System;
 using System.Text.Json;
 using System.Threading.Tasks;
 using Gott.Models;
+using Gott.Extensions;
 using Gott.Services;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
@@ -15,14 +16,14 @@ public static class CronjobEndpoint
     {
         app.MapGet("/api/cronjobs", async (HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var cronjobs = await db.GetCronjobs(userId);
             return Results.Ok(cronjobs);
         });
 
         app.MapGet("/api/cronjobs/{id}", async (string id, HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var cronjob = await db.GetCronjob(id);
             if (cronjob == null || cronjob.UserId != userId) return Results.NotFound();
             return Results.Ok(cronjob);
@@ -30,7 +31,7 @@ public static class CronjobEndpoint
 
         app.MapPost("/api/cronjobs", async (HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var body = await JsonSerializer.DeserializeAsync<JsonElement>(ctx.Request.Body);
 
             var name = body.TryGetProperty("name", out var n) ? n.GetString() ?? "" : "";
@@ -79,7 +80,7 @@ public static class CronjobEndpoint
 
         app.MapPut("/api/cronjobs/{id}", async (string id, HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var existing = await db.GetCronjob(id);
             if (existing == null || existing.UserId != userId) return Results.NotFound();
 
@@ -134,7 +135,7 @@ public static class CronjobEndpoint
 
         app.MapDelete("/api/cronjobs/{id}", async (string id, HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var existing = await db.GetCronjob(id);
             if (existing == null || existing.UserId != userId) return Results.NotFound();
 
@@ -144,7 +145,7 @@ public static class CronjobEndpoint
 
         app.MapPost("/api/cronjobs/{id}/toggle", async (string id, HttpContext ctx, DatabaseService db) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var existing = await db.GetCronjob(id);
             if (existing == null || existing.UserId != userId) return Results.NotFound();
 
@@ -158,7 +159,7 @@ public static class CronjobEndpoint
 
         app.MapPost("/api/cronjobs/{id}/run", async (string id, HttpContext ctx, DatabaseService db, CronjobService cronjobService) =>
         {
-            var userId = ctx.Items["UserId"]?.ToString();
+            var userId = ctx.GetUserId();
             var existing = await db.GetCronjob(id);
             if (existing == null || existing.UserId != userId) return Results.NotFound();