Pending count queries included NULL-ID rows that _get_next_item filters
out, so pending stayed > 0 even when no processable items remained.
Workers reported running instead of idle, UI never turned green. Added
AND id IS NOT NULL to _count_pending_items across all 9 workers to
match the _get_next_item filter.
Workers would endlessly match the same track because UPDATE WHERE id =
NULL matches 0 rows in SQL. Added AND id IS NOT NULL to all enrichment
queries (individual, batch EXISTS, and batch fetch) across all 9
workers. Also added process-level guard for belt-and-suspenders safety.
Fix Deezer get_track → get_track_details method name mismatch.
