Forward Proxmox endpoint tuning to backend

[emersonfelipesp]

Refs emersonfelipesp/proxbox-api#211

Summary

• Include Proxmox endpoint verify_ssl, timeout, max_retries, and retry_backoff in the payload sent to proxbox-api before live reads.
• Coerce numeric endpoint tuning values to JSON-safe int/float values.
• Document the backend_sync payload behavior in the scoped views guide.

Tests

• uv run pytest tests/test_backend_sync_placement.py
• uv run ruff check netbox_proxbox/views/backend_sync.py tests/test_backend_sync_placement.py
• uv run ruff format --check netbox_proxbox/views/backend_sync.py tests/test_backend_sync_placement.py