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/tests/test_discovery.py

@@ -72,6 +72,21 @@ def test_unhealthy_source_contributes_nothing(conn):
     assert _artist_rows(conn) == []
 
 
+def test_source_with_raising_health_is_skipped_not_fatal(conn):
+    insert_watched_artist(conn, mbid="s1", name="Seed")
+
+    class BoomSource:
+        name = "boom"
+        def health(self):
+            raise RuntimeError("boom")
+        def similar_artists(self, mbid):
+            return []
+
+    good = FakeSimilaritySource(similar={"s1": [SimilarArtist("c1", "X", 0.9)]})
+    result = run_discovery(conn, [BoomSource(), good], FakeMbBrowser(), DiscoveryConfig())
+    assert result.artists == 1  # good source still processed; boom skipped, no crash
+
+
 def test_config_from_config_parses_and_defaults():
     cfg = DiscoveryConfig.from_config({"discover.enabled": "true", "discover.maxSeeds": "10",
                                        "discover.minScore": "0.3"})