feat: first-class SponsorBlock toggle

A "Remove sponsor segments" switch next to "Split by chapters" queues
the download with the same postprocessor pair the CLI's
--sponsorblock-remove sponsor builds (SponsorBlock + ModifyChapters).
The flag persists as a cookie like the other form options, survives in
the queue records, and is carried into retries.

The pair is registered above the chapter-splitting block: yt-dlp runs
same-stage postprocessors in list order, so ModifyChapters has to
rewrite the chapter list before FFmpegSplitChapters cuts the file up,
matching what the CLI builds for --sponsorblock-remove sponsor
--split-chapters. With both toggles on the other way around the chapter
files keep the sponsor segments and the removal desyncs the remaining
chapter timings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/main.py

@@ -725,6 +725,7 @@ def parse_download_options(post: dict) -> dict:
     playlist_item_limit = post.get('playlist_item_limit')
     auto_start = post.get('auto_start')
     split_by_chapters = post.get('split_by_chapters')
+    sponsorblock = bool(post.get('sponsorblock'))
     chapter_template = post.get('chapter_template')
     subtitle_language = post.get('subtitle_language')
     subtitle_mode = post.get('subtitle_mode')
@@ -845,6 +846,7 @@ def parse_download_options(post: dict) -> dict:
         'playlist_item_limit': playlist_item_limit,
         'auto_start': auto_start,
         'split_by_chapters': split_by_chapters,
+        'sponsorblock': sponsorblock,
         'chapter_template': chapter_template,
         'subtitle_language': subtitle_language,
         'subtitle_mode': subtitle_mode,
@@ -890,6 +892,7 @@ async def add(request):
         o['ytdl_options_overrides'],
         o['clip_start'],
         o['clip_end'],
+        sponsorblock=o['sponsorblock'],
     )
     return web.Response(text=serializer.encode(status))