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>

app/scheduler.py

@@ -6,6 +6,8 @@ from apscheduler.triggers.interval import IntervalTrigger
 from app.mail_processor import process_mailbox
 from app.smb_processor import process_smb_share
 from app.amazon_processor import process_amazon
+from app.amazon_api import process_amazon_api
+from app.database import get_settings
 
 logger = logging.getLogger(__name__)
 
@@ -34,7 +36,12 @@ async def _run_processor():
         # Amazon separately with timeout - must not block next scheduler runs
         logger.info("Starte automatische Amazon-Verarbeitung...")
         try:
-            amazon_result = await asyncio.wait_for(process_amazon(), timeout=300)
+            settings = await get_settings()
+            amazon_mode = settings.get("amazon_mode", "browser")
+            if amazon_mode == "api":
+                amazon_result = await asyncio.wait_for(process_amazon_api(), timeout=300)
+            else:
+                amazon_result = await asyncio.wait_for(process_amazon(), timeout=300)
             logger.info(f"Amazon-Verarbeitung abgeschlossen: {amazon_result}")
         except asyncio.TimeoutError:
             logger.error("Amazon-Verarbeitung nach 5 Minuten abgebrochen (Timeout)")