fix(scanner): increase watcher channel buffers to prevent dropped filesystem events

When files were moved between libraries, the small channel buffers (size 1)
throughout the watcher pipeline caused backpressure that led to dropped
filesystem events. This meant only some of the affected folders were scanned,
preventing cross-library move detection from working correctly.

Increase all watcher channel buffers to 500 and switch to blocking sends
to ensure no filesystem events are silently dropped.

scanner/watcher.go

@@ -48,7 +48,7 @@ func GetWatcher(ds model.DataStore, s model.Scanner) Watcher {
 			ds:              ds,
 			scanner:         s,
 			triggerWait:     conf.Server.Scanner.WatcherWait,
-			watcherNotify:   make(chan scanNotification, 1),
+			watcherNotify:   make(chan scanNotification, 500),
 			libraryWatchers: make(map[int]*libraryWatcherInstance),
 		}
 	})
@@ -272,12 +272,8 @@ func (w *watcher) processLibraryEvents(ctx context.Context, lib *model.Library,
 				continue
 			}
 
-			// Notify the main watcher of changes
-			select {
-			case w.watcherNotify <- scanNotification{Library: lib, FolderPath: folderPath}:
-			default:
-				// Channel is full, notification already pending
-			}
+			// Notify the main watcher of changes. This will trigger a scan after the debounce period.
+			w.watcherNotify <- scanNotification{Library: lib, FolderPath: folderPath}
 		}
 	}
 }