fix(worker): run discovery at most once per loop iteration

The scheduled-tick branch and the discover.requested one-shot branch
both read the same stale per-iteration config snapshot, so an enabled+due
sweep coinciding with a pending request ran discovery twice back-to-back
(idempotent but wasteful). Extract maybe_run_discovery, which unifies both
triggers into a single run and is now unit-tested.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/tests/test_discovery_trigger.py

@@ -1,5 +1,7 @@
+from lyra_worker import main
 from lyra_worker.adapters.fakes import FakeMbBrowser, FakeSimilaritySource
-from lyra_worker.main import _run_discovery
+from lyra_worker.discovery import DiscoveryConfig
+from lyra_worker.main import _run_discovery, maybe_run_discovery
 from lyra_worker.similarity.base import SimilarArtist
 from tests.conftest import insert_watched_artist
 
@@ -33,3 +35,44 @@ def test_run_discovery_writes_result_and_clears_flag(conn):
     with conn.cursor() as cur:
         cur.execute('SELECT count(*) FROM "DiscoverySuggestion" WHERE kind = \'artist\'')
         assert cur.fetchone()[0] == 1
+
+
+def test_maybe_run_discovery_runs_once_when_due_and_requested(conn, monkeypatch):
+    # A scheduled tick that is due AND a pending one-shot request in the same loop
+    # iteration must trigger discovery exactly once, not twice.
+    calls = []
+    monkeypatch.setattr(main, "_run_discovery", lambda *a, **k: calls.append(1))
+    config = {"discover.enabled": "true", "discover.requested": "true"}
+    dcfg = DiscoveryConfig.from_config(config)
+    new_tick = maybe_run_discovery(
+        conn, [], FakeMbBrowser(), config, dcfg, now=10_000.0, last_discover_tick=0.0
+    )
+    assert calls == [1]           # exactly one run
+    assert new_tick == 10_000.0   # schedule tick advanced
+
+
+def test_maybe_run_discovery_requested_when_disabled_does_not_advance_tick(conn, monkeypatch):
+    # A one-shot request runs even when discovery is disabled, but must not advance the
+    # schedule tick (there is no active schedule).
+    calls = []
+    monkeypatch.setattr(main, "_run_discovery", lambda *a, **k: calls.append(1))
+    config = {"discover.enabled": "false", "discover.requested": "true"}
+    dcfg = DiscoveryConfig.from_config(config)
+    new_tick = maybe_run_discovery(
+        conn, [], FakeMbBrowser(), config, dcfg, now=10_000.0, last_discover_tick=42.0
+    )
+    assert calls == [1]
+    assert new_tick == 42.0        # unchanged
+
+
+def test_maybe_run_discovery_idle_does_nothing(conn, monkeypatch):
+    calls = []
+    monkeypatch.setattr(main, "_run_discovery", lambda *a, **k: calls.append(1))
+    config = {"discover.enabled": "true", "discover.requested": "false"}
+    dcfg = DiscoveryConfig.from_config(config)
+    # enabled but not yet due (tick was recent relative to now)
+    new_tick = maybe_run_discovery(
+        conn, [], FakeMbBrowser(), config, dcfg, now=100.0, last_discover_tick=99.0
+    )
+    assert calls == []
+    assert new_tick == 99.0