Load system prompt from persistent file

- Load system prompt from data/system-prompt.txt (persistent)
- Fallback to AI_SYSTEM_PROMPT env variable
- Fallback to hardcoded default
- File can be edited without rebuilding container
- Includes default UlfBot/SchrottBot personality

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -4,4 +4,5 @@ BOT_TOKEN=your_telegram_bot_token_here
 AI_DEFAULT_API_KEY=your_openai_api_key
 AI_DEFAULT_BASE_URL=https://api.openai.com/v1
 AI_DEFAULT_MODEL=gpt-4o-mini
+# System prompt can also be loaded from data/system-prompt.txt
 AI_SYSTEM_PROMPT=Du bist ein freundlicher Chat-Bot in einer Telegram-Gruppe. Antworte kurz und prägnant auf Deutsch.