feat: add configurable multi-threaded parallel scanning for library tasks

JellyfinPlugin/Tasks/LibraryRetroactiveScanTask.cs

@@ -42,19 +42,28 @@ namespace JellyfinPlugin.Tasks
                 .ToList();
 
             int total = items.Count;
+            if (total == 0) return;
+
             int processed = 0;
+            int maxParallel = Math.Clamp(config.MaxParallelScans, 1, 16);
 
-            foreach (var item in items)
+            var parallelOptions = new ParallelOptions
             {
-                cancellationToken.ThrowIfCancellationRequested();
+                MaxDegreeOfParallelism = maxParallel,
+                CancellationToken = cancellationToken
+            };
 
+            _logger.LogInformation("Starting Retroactive Scan of {Total} items with {MaxParallel} parallel workers.", total, maxParallel);
+
+            await Parallel.ForEachAsync(items, parallelOptions, async (item, ct) =>
+            {
                 if (item.ProviderIds != null && 
                     item.ProviderIds.TryGetValue("BlackBarCrop", out string? existingCrop) && 
                     !string.IsNullOrEmpty(existingCrop))
                 {
-                    processed++;
-                    progress.Report((processed / (double)total) * 100);
-                    continue;
+                    int currentSkipped = Interlocked.Increment(ref processed);
+                    progress.Report((currentSkipped / (double)total) * 100);
+                    return;
                 }
 
                 var path = item.Path;
@@ -71,7 +80,7 @@ namespace JellyfinPlugin.Tasks
                                 item.ProviderIds = new Dictionary<string, string>();
                             }
                             item.ProviderIds["BlackBarCrop"] = cropData;
-                            await _libraryManager.UpdateItemAsync(item, item, ItemUpdateType.MetadataEdit, cancellationToken);
+                            await _libraryManager.UpdateItemAsync(item, item, ItemUpdateType.MetadataEdit, ct);
                             _logger.LogInformation("Found black bars {Crop} in {Path}", cropData, path);
                         }
                     }
@@ -81,9 +90,9 @@ namespace JellyfinPlugin.Tasks
                     }
                 }
 
-                processed++;
-                progress.Report((processed / (double)total) * 100);
-            }
+                int currentDone = Interlocked.Increment(ref processed);
+                progress.Report((currentDone / (double)total) * 100);
+            });
         }
 
         public IEnumerable<TaskTriggerInfo> GetDefaultTriggers()