fix(utils): log fire-and-forget background task failures

[kiwipaulrob]

Problem

create_task() in sendspin/utils.py registers a done-callback that retrieves the exception (suppressing "never retrieved" warnings) and then discards it with no logging. Every fire-and-forget failure in the daemon is invisible, including the external-volume path (hook/amixer/PulseAudio errors swallowed, volume never published).

Fix

Replaces the lambda with a _log_task_exception callback that logs the exception with the task name (exc_info preserved). Cancelled tasks stay silent, as before; behaviour is otherwise unchanged.

Verification

• Full suite (pytest tests/, excluding serve + audio_devices): 123 passed
• ruff check clean

Small, no behaviour change beyond visibility. Happy to lower it to debug level if error is too noisy.