Amazon Business API integration replacing browser automation

- Add amazon_api.py with Reconciliation + Document API client
- OAuth flow with manual code exchange for local installations
- Dual mode: API (recommended) or Browser automation (fallback)
- New settings: amazon_app_id, amazon_client_id, amazon_client_secret, amazon_refresh_token
- Platform UI with mode switcher, API credential fields, OAuth button
- Scheduler supports both API and browser modes
- README with full Amazon API setup guide
- httpx added for async HTTP requests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -10,4 +10,5 @@ services:
       - DB_PATH=/data/belegimport.db
       - TZ=Europe/Berlin
       - LOG_LEVEL=DEBUG
+      - OAUTH_REDIRECT_BASE=https://hacker-net.de
     restart: unless-stopped