fix: show the post-download processing phase in the UI (closes #424)

yt-dlp reports a download 'finished' as soon as the media bytes have
landed, but the ffmpeg work that follows -- merging, re-encoding, chapter
splitting, sponsor removal -- routinely takes longer than the download
itself. The postprocessor hook only ever looked at MoveFiles and
SplitChapters finishing, so that whole phase said nothing: the row sat in
the Downloading table on a full, frozen progress bar, and the item counted
as neither active nor queued in the header stats.

Report a 'postprocessing' status when a postprocessor starts, and reuse
the indeterminate bar the UI already runs for 'preparing', labelled so a
long re-encode is distinguishable from a stall.

Measured on a real download re-encoded with the reporter's FFmpegCopyStream
config, the UI now receives:

    [ 0.27s] downloading      moving bar
    [ 0.28s] finished         <- yt-dlp, bytes are down
    [ 0.28s] postprocessing   animated "Post-processing"
    [13.18s] finished         done

i.e. 12.9s that used to render as a static 100% bar.

The hook is latched off once MoveFiles reports finished, since that branch
announces the finished file: a 'postprocessing' arriving afterwards would
flip the row back out of its completed state for no reason. It cannot fail
the download -- _download puts an unconditional 'finished' once download()
returns, so the terminal status is settled either way -- but the flicker
and the extra broadcast are both pointless. yt-dlp does run an 'after_move'
stage after MoveFiles, though every postprocessor MeTube configures is
'after_filter' or 'post_process', both of which precede it.

update_status drops a repeated 'postprocessing': yt-dlp's metaclass wraps
run() once per class in a postprocessor's MRO, so one whose subclass
overrides run reports started twice -- FFmpegCopyStream did exactly that in
the live run, three queued statuses collapsing to a single broadcast.

Extracted to _make_postprocessor_hook, mirroring _make_progress_hook, so
the ordering above is testable; every new regression test was confirmed to
fail with the fix backed out.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/app/app.html

@@ -732,8 +732,12 @@
                     }
                   </span>
                 } @else {
-                  <ngb-progressbar height="1.5rem" [showValue]="download.value.status !== 'preparing'" [striped]="download.value.status === 'preparing'" [animated]="download.value.status === 'preparing'" type="success"
-                  [value]="download.value.status === 'preparing' ? 100 : download.value.percent" class="download-progressbar" />
+                  <ngb-progressbar height="1.5rem" [showValue]="!isIndeterminate(download.value)" [striped]="isIndeterminate(download.value)" [animated]="isIndeterminate(download.value)" type="success"
+                  [value]="isIndeterminate(download.value) ? 100 : download.value.percent" class="download-progressbar">
+                    @if (download.value.status === 'postprocessing') {
+                      <span>Post-processing</span>
+                    }
+                  </ngb-progressbar>
                 }
               </div>
             </td>

ui/src/app/app.spec.ts

@@ -473,4 +473,50 @@ describe('App', () => {
       expect(app.buildChapterDownloadLink(audio, 'ch1.mp3')).toBe('audio_download/ch1.mp3');
     });
   });
+
+  // Issue #424: ffmpeg work after the bytes land (merge, re-encode, split) used
+  // to leave the row on a full, frozen bar with the item counted as neither
+  // active nor queued.
+  describe('post-processing is visible (#424)', () => {
+    const queueEntry = (status: string): Download => ({
+      id: 'vid1',
+      title: 'Test',
+      url: 'https://example.com/v',
+      download_type: 'video',
+      quality: 'best',
+      format: 'any',
+      folder: '',
+      custom_name_prefix: '',
+      playlist_item_limit: 0,
+      status,
+      msg: '',
+      percent: 100,
+      speed: 0,
+      eta: 0,
+      filename: '',
+      checked: false,
+    } as Download);
+
+    it('runs the bar indeterminate while preparing or post-processing', () => {
+      const app = TestBed.createComponent(App).componentInstance;
+      expect(app.isIndeterminate(queueEntry('preparing'))).toBe(true);
+      expect(app.isIndeterminate(queueEntry('postprocessing'))).toBe(true);
+      expect(app.isIndeterminate(queueEntry('downloading'))).toBe(false);
+      expect(app.isIndeterminate(queueEntry('pending'))).toBe(false);
+    });
+
+    it('labels the bar and counts the item as active', () => {
+      // The component subscribes to queueChanged on construction, so the entry
+      // has to be announced after it exists or updateMetrics never runs.
+      const fixture = TestBed.createComponent(App);
+      downloads.queue.set('https://example.com/v', queueEntry('postprocessing'));
+      downloads.queueChanged.next();
+      fixture.detectChanges();
+
+      expect((fixture.nativeElement as HTMLElement).textContent).toContain('Post-processing');
+      expect(fixture.componentInstance.activeDownloads).toBe(1);
+      expect(fixture.componentInstance.queuedDownloads).toBe(0);
+    });
+  });
+
 });

ui/src/app/app.ts

@@ -1184,6 +1184,14 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
     this.downloads.startById([id]).subscribe((res) => this.handleActionResult(res, 'Start download failed'));
   }
 
+  // 'preparing' (yt-dlp starting up) and 'postprocessing' (ffmpeg merging,
+  // re-encoding or splitting once the bytes have landed) both have real work in
+  // flight with no percentage to report, so the bar runs animated at full width
+  // instead of showing a number that cannot move.
+  isIndeterminate(download: Download): boolean {
+    return download.status === 'preparing' || download.status === 'postprocessing';
+  }
+
   liveCountdownSeconds(download: Download): number | null {
     const ts = download.live_release_timestamp;
     if (ts == null || download.status !== 'scheduled') {
@@ -1697,7 +1705,7 @@ export class App implements AfterViewInit, OnInit, OnDestroy {
       if (download.status === 'downloading') {
         active++;
         speed += download.speed || 0;
-      } else if (download.status === 'preparing') {
+      } else if (download.status === 'preparing' || download.status === 'postprocessing') {
         active++;
       } else if (download.status === 'pending' || download.status === 'scheduled') {
         queued++;