Add all environment variables to docker-compose

Pass all .env settings to container:
- AI_DEFAULT_API_KEY
- AI_DEFAULT_BASE_URL
- AI_DEFAULT_MODEL
- AI_SYSTEM_PROMPT

With sensible defaults for optional values.

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

docker-compose.yml

@@ -5,6 +5,10 @@ services:
     restart: unless-stopped
     environment:
       - BOT_TOKEN=${BOT_TOKEN}
+      - AI_DEFAULT_API_KEY=${AI_DEFAULT_API_KEY:-}
+      - AI_DEFAULT_BASE_URL=${AI_DEFAULT_BASE_URL:-https://api.openai.com/v1}
+      - AI_DEFAULT_MODEL=${AI_DEFAULT_MODEL:-gpt-4o-mini}
+      - AI_SYSTEM_PROMPT=${AI_SYSTEM_PROMPT:-Du bist ein freundlicher Chat-Bot in einer Telegram-Gruppe. Antworte kurz und prägnant auf Deutsch.}
     volumes:
       - ulfbot-data:/app/data