feat: Krea Image Generation Endpoint

- KreaService: GenerateAsync() mit Job-Polling, Image-Download, Metadaten-Speicherung
- KreaEndpoint: POST /api/images/generate, GET /api/images, GET /api/images/file/{filename}, GET /api/images/status
- KreaImageRecord: Output-Model
- Output: data/images/<timestamp>-<index>.<ext> + data/images/<timestamp>.json
- 16 Modelle: flux, flux-kontext, flux-1.1-pro, imagen-3/4, chatgpt-image, runway-gen-4, etc.
- appsettings.json: KreaToken (Format: KEY_ID:SECRET), obsolete TelegramBotToken/WebhookSecret entfernt

appsettings.json

@@ -9,5 +9,6 @@
   "AnthropicToken": "",
   "MaxIterations": 100,
   "PayloadLogging": false,
-  "PayloadLoggingGzip": false
+  "PayloadLoggingGzip": false,
+  "KreaToken": ""
 }