a15f4ae8bb
Adds registry.build_similarity_sources() (always constructs ListenBrainzSource; reachability is a per-run health() concern, not a startup gate) and wires discovery into the worker's main loop: a scheduled sweep gated on discover.enabled + DISCOVER_TICK_SECONDS, and a one-shot discover.requested trigger mirroring the existing scan.requested pattern. Also hardens run_discovery's health-check pass: a source whose health() raises is now logged and skipped instead of aborting the whole sweep (accepted finding from the Task-3 review). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>