feat: implement ProcessVerarbeiteteDocuments (Upload-Check)

Ported ProcessVerarbeiteteDocuments() from C# ProcessUploads.cs:
- Checks docs tagged "hochgeladen" → eingangsrechnungVorhanden()
- On match: livesearch, update title/type/created/correspondent/tags,
  set custom fields (externeBelegnummer, AgrarmonitorLink), addNote
- Tag "hochgeladen" → "fertig" swap; owner via Client.AgrarmonitorBetriebId
- 401/403 guard: clearClient() + break (same pattern as runPolling)
- Cron: AGRARMONITOR_UPLOAD_CHECK_CRON (default: 0 * * * * *)
- New settings: agrarmonitor_tag_hochgeladen, agrarmonitor_link_field
- Endpoint: POST /api/agrarmonitor/process-uploads
- Frontend: polling-config extended with tagHochgeladen + linkField select,
  new card "Dokumenten-Verarbeitung" with run button + result display

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

docker-compose.yml

@@ -46,6 +46,7 @@ services:
       - AGRARMONITOR_COOKIE_PATH=${AGRARMONITOR_COOKIE_PATH:-./data/agrarmonitor-cookies.json}
       - AGRARMONITOR_ENCRYPTION_KEY=${AGRARMONITOR_ENCRYPTION_KEY:-}
       - AGRARMONITOR_POLLING_CRON=${AGRARMONITOR_POLLING_CRON:-}
+      - AGRARMONITOR_UPLOAD_CHECK_CRON=${AGRARMONITOR_UPLOAD_CHECK_CRON:-}
     volumes:
       - /mnt/scans:/mnt/scans
       - /mnt/paperlessmanager:/mnt/data