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/database.py

@@ -10,7 +10,7 @@ logger = logging.getLogger(__name__)
 
 _fernet = None
 
-ENCRYPTED_KEYS = {"imap_password", "smtp_password", "smb_password", "amazon_password"}
+ENCRYPTED_KEYS = {"imap_password", "smtp_password", "smb_password", "amazon_password", "amazon_client_secret", "amazon_refresh_token"}
 
 DEFAULT_SETTINGS = {
     "imap_server": "",
@@ -53,6 +53,12 @@ DEFAULT_SETTINGS = {
     "amazon_domain": "amazon.de",
     "amazon_last_sync": "",
     "amazon_since_date": "",
+    # Amazon API (SP-API / Business API)
+    "amazon_app_id": "",  # amzn1.sp.solution.xxxxx (from Developer Portal)
+    "amazon_client_id": "",  # amzn1.application-oa2-client.xxxxx (LWA Client ID)
+    "amazon_client_secret": "",  # LWA Client Secret
+    "amazon_refresh_token": "",
+    "amazon_mode": "browser",  # "browser" or "api"
     # Debug
     "debug_save_amazon_pdfs": "false",
 }