refactor: split proxmox.py (4676 lines) into package with 7 modules

- _helpers.py: shared helpers (check_vm_access, _get_proxmox_client, etc.)
- servers.py: HTML pages, server CRUD, resources, node status (16 routes)
- sdn.py: SDN zones/vnets/subnets (10 routes)
- snapshots.py: VM/container snapshots + archives (10 routes)
- vms.py: VM/LXC control, HA, monitoring, VNC, terminal, bulk ops (42 routes)
- tasks.py: Proxmox task management (6 routes)
- __init__.py: re-exports unified router (84 routes total, no public API change)

https://claude.ai/code/session_019rK2wvaWg5zXFwmAPPEB7R

backend/app/api/proxmox/__init__.py

@@ -0,0 +1,13 @@
+from fastapi import APIRouter
+from .servers import router as servers_router
+from .sdn import router as sdn_router
+from .snapshots import router as snapshots_router
+from .vms import router as vms_router
+from .tasks import router as tasks_router
+
+router = APIRouter()
+router.include_router(servers_router)
+router.include_router(sdn_router)
+router.include_router(snapshots_router)
+router.include_router(vms_router)
+router.include_router(tasks_router)