When an Xtream Codes provider returns no live streams on a routine refresh
(a transient upstream failure, a fetch exception, or no enabled category
matching), collect_xc_streams() returns an empty list. The refresh then fell
through to stale-marking and sync_auto_channels, which -- with nothing seen
this refresh -- deletes the account's entire auto-created channel lineup via
its per-group "no streams remaining" branch.
Guard the XC branch of _refresh_single_m3u_account_impl: on an empty result,
set the account to ERROR and return before stale-marking and auto-sync,
mirroring the standard-path empty/failed-download guards already in the
function. A transient empty fetch can no longer destroy channels.
Adds apps/m3u/tests/test_xc_empty_fetch_guard.py covering both the abort
(sync not called, channels preserved, streams not marked stale, status ERROR)
and the healthy path (non-empty fetch still runs sync).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
