Replace the expensive DISTINCT query for task filter dropdowns with a
dedicated TaskIdentifier registry table backed by Redis. Environments
migrate automatically on their next deploy, with a transparent fallback
to the legacy query for unmigrated environments. Also fixes duplicate
dropdown entries when a task changes trigger source, and adds
active/archived grouping for removed tasks. Moves BackgroundWorkerTask
reads in the trigger hot path to the read replica.
Eric Allam