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/discovery.py

@@ -48,6 +48,19 @@ class DiscoveryResult:
     albums: int
 
 
+def _healthy_sources(sources):
+    """Return sources whose health() is truthy; a source whose health() raises
+    is logged and skipped, never aborting the sweep."""
+    live = []
+    for s in sources:
+        try:
+            if s.health():
+                live.append(s)
+        except Exception as e:  # a bad health check must not abort the sweep
+            print(f"worker: discovery source {getattr(s, 'name', '?')} health check failed: {e}", flush=True)
+    return live
+
+
 def _seed_artists(conn: psycopg.Connection, cfg: DiscoveryConfig):
     with conn.cursor() as cur:
         cur.execute(
@@ -134,7 +147,7 @@ def run_discovery(conn: psycopg.Connection, sources: list[SimilaritySource],
     """Aggregate similar artists across the library's seed artists; upsert suggestions."""
     seeds = _seed_artists(conn, cfg)
     followed = _followed_mbids(conn)
-    live = [s for s in sources if s.health()]
+    live = _healthy_sources(sources)
 
     agg: dict[str, dict] = {}
     for seed_mbid, _seed_name in seeds: