fix(scanner): resolve data race on conf.Server access in getScanner

Captured DevExternalScanner config value in the controller struct at
construction time instead of reading the global conf.Server pointer in
getScanner(). The background goroutine spawned by ScanFolders() was
reading conf.Server.DevExternalScanner concurrently with test cleanup
reassigning the conf.Server pointer, causing a data race detected by
the race detector in the E2E test suite.

scanner/controller.go

@@ -35,15 +35,16 @@ func New(rootCtx context.Context, ds model.DataStore, cw artwork.CacheWarmer, br
 		broker:             broker,
 		pls:                pls,
 		metrics:            m,
+		devExternalScanner: conf.Server.DevExternalScanner,
 	}
-	if !conf.Server.DevExternalScanner {
+	if !c.devExternalScanner {
 		c.limiter = P(rate.Sometimes{Interval: conf.Server.DevActivityPanelUpdateRate})
 	}
 	return c
 }
 
 func (s *controller) getScanner() scanner {
-	if conf.Server.DevExternalScanner {
+	if s.devExternalScanner {
 		return &scannerExternal{}
 	}
 	return &scannerImpl{ds: s.ds, cw: s.cw, pls: s.pls}
@@ -99,6 +100,7 @@ type controller struct {
 	metrics            metrics.Metrics
 	pls                core.Playlists
 	limiter            *rate.Sometimes
+	devExternalScanner bool
 	count              atomic.Uint32
 	folderCount        atomic.Uint32
 	changesDetected    bool