feat(discovery): build_similarity_sources + worker sweep/trigger wiring

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>

worker/lyra_worker/registry.py

@@ -12,6 +12,8 @@ from lyra_worker.adapters.youtube import YouTubeAdapter
 from lyra_worker.browser import MbBrowser
 from lyra_worker.probe import AudioProbe
 from lyra_worker.resolver import MbResolver
+from lyra_worker.similarity._listenbrainz import ListenBrainzSource
+from lyra_worker.similarity.base import SimilaritySource
 from lyra_worker.tagger import Tagger
 
 
@@ -47,3 +49,9 @@ def build_browser() -> MbBrowser:
 def build_probe() -> AudioProbe:
     """The audio probe used by the library scan."""
     return MutagenProbe()
+
+
+def build_similarity_sources(config: dict) -> list[SimilaritySource]:
+    """Similarity sources for discovery. ListenBrainz needs no credentials, so it is
+    always constructed; per-run reachability is checked via each source's health()."""
+    return [ListenBrainzSource(base_url=config.get("discover.listenBrainzUrl") or "")]