Hub-and-Spoke Umbau: Multi-Tenant Zentrale + Satellite-Agent

- Backend: Customer/Satellite Models, customer_id auf Server/Job/Audit
- Satellite-API: heartbeat, poll (atomares Claiming), logs, result,
  scan-result, health-report - Auth via X-Api-Key (SHA-256 gehasht)
- Job-Queue: pending/claimed/running/success/failed + Stale-Janitor
- Batch-Trigger: ein Job pro Server, Satellite arbeitet sequenziell ab
- Credentials bleiben lokal: nur symbolische credential_ref zentral
- Neues Paket satellite/: Pull-Loop, WinRM/SSH/CAU/Scanner, PyInstaller-tauglich
- Frontend: Kunden-Switcher, Satelliten-View, Polling statt WebSocket
- Entfernt: WebSocket/Socket.io, Redis, zentrale Credentials, JobRunner
- Docs: README/AGENTS/PROMPT auf neue Architektur aktualisiert

backend/app/models/__init__.py

@@ -1,14 +1,16 @@
 """SQLAlchemy ORM models."""
 
 from app.models.audit_log import AuditLog
-from app.models.credential import Credential
+from app.models.customer import Customer
+from app.models.satellite import Satellite
 from app.models.server import Server
 from app.models.update_job import UpdateJob, UpdateLog
 from app.models.user import User
 
 __all__ = [
     "AuditLog",
-    "Credential",
+    "Customer",
+    "Satellite",
     "Server",
     "UpdateJob",
     "UpdateLog",