feat(#392): scan run lifecycle events on the SSE bus

Slice 3c — completes the server-side producer set.

Publishes scan.run_started when InsertScanRun succeeds and
scan.run_finished after FinishScanRun completes (success or with an
error_message payload). Per-file progress events are intentionally NOT
emitted — they'd flood the stream during large library scans without
giving the admin scan card anything actionable. The two-beat signal
gives the UI enough to invalidate scanStatusProvider at the right
moments.

Bus access uses a package-level setter on internal/library because
threading bus through RunScan + TryStartScan + Scheduler + all their
callers would touch ~10 sites without changing behavior at the boundaries
that don't publish. Per-process singleton, matches the log.SetDefault
idiom. cmd/minstrel/main.go calls library.SetEventBus(bus) once at
startup; test contexts that never call it skip publishing safely
(publishScanEvent is a no-op when bus is nil).

This completes the server side of #392. Slice 4 wires the Flutter
consumer: live_events_provider.dart (StreamProvider) +
live_events_dispatcher.dart (event-kind → provider invalidation)
+ AppLifecycleState cold-start invalidation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -152,6 +152,10 @@ func run() error {
 	// Constructed before any service that publishes so they all share the
 	// same instance.
 	bus := eventbus.New()
+	// Scan-run lifecycle events use a package-level setter rather than
+	// threading the bus through RunScan + TryStartScan + the Scheduler
+	// + every test caller. Per-process singleton, set once at startup.
+	library.SetEventBus(bus)
 	lidarrReconciler := lidarrrequests.NewReconciler(pool, lidarrCfg, logger.With("component", "lidarr"), bus)
 	go lidarrReconciler.Run(ctx)