Fix RuntimeError when unlinking Spoolman spool from holder modal

_moonraker_set_active_spool used asyncio.ensure_future which requires
a running event loop in the current thread.  Sync FastAPI endpoints run
in an AnyIO worker thread that has no event loop, so unlinking the SP
slot (which triggers CLEAR_ACTIVE_SPOOL in moonraker mode) crashed with
RuntimeError: There is no current event loop in thread.

Replace the async wrapper + ensure_future with a plain daemon thread.
_moonraker_send_gcode is already a blocking HTTP call, so no executor
indirection is needed — a thread achieves the same fire-and-forget goal
while working from both async and sync call sites.

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