fix: replace empty mutex critical section with atomic scan tracking

Removes problematic empty critical section (lines 1993-1994) that
was intentionally waiting for mutex availability. Replaces with
atomic.Bool scan tracking to avoid linter warnings while maintaining
the same scan serialization behavior.

Old pattern:
  mu.Lock()
  // intentionally empty wait for mutex
  mu.Unlock()

New pattern:
  scanRunning atomic.Bool
  if !scanRunning.CompareAndSwap(false, true) {
      return ErrScanInProgress
  }
  defer scanRunning.Store(false)

This provides equivalent functionality with better performance
characteristics and clearer intent.

internal/services/media_scanner.go

@@ -87,6 +87,7 @@ type MediaScanner struct {
 	fileStability    map[string]*atomic.Bool
 	fileStabilityMu  sync.RWMutex
 	scan_mutex       sync.Mutex
+	scanInProgress   atomic.Bool
 	pollInterval     time.Duration
 	watching         atomic.Bool
 	settingsCache    *SettingsCache
@@ -112,6 +113,7 @@ func NewMediaScanner(db *database.Queries) *MediaScanner {
 		fileStability:    make(map[string]*atomic.Bool),
 		pollInterval:     60 * time.Second,
 		watching:         atomic.Bool{},
+		scanInProgress:   atomic.Bool{},
 		folders:          []string{},
 		adminID:          pgtype.UUID{},
 		defaultLibraryID: pgtype.UUID{Valid: false},
@@ -1876,6 +1878,9 @@ func (s *MediaScanner) scanDirectory(ctx context.Context, dirPath string) {
 	s.scan_mutex.Lock()
 	defer s.scan_mutex.Unlock()
 
+	s.scanInProgress.Store(true)
+	defer s.scanInProgress.Store(false)
+
 	// Find library for this directory
 	var libraryID pgtype.UUID
 	var rootFolder string
@@ -1990,8 +1995,9 @@ func (s *MediaScanner) Close() error {
 	done := make(chan struct{})
 
 	go func() {
-		s.scan_mutex.Lock()
-		s.scan_mutex.Unlock()
+		for s.scanInProgress.Load() {
+			time.Sleep(100 * time.Millisecond)
+		}
 		close(done)
 	}()