feat: carry the SponsorBlock toggle into subscriptions

Subscriptions download unattended, which is where skipping sponsor reads
is most useful, so the flag now travels the same path the other download
options take: stored on SubscriptionInfo, persisted in the record, and
passed to add_entry for every entry a check queues.

Like the clip bounds, it is set when the subscription is created; the
update endpoint's field list is unchanged.

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

app/main.py

@@ -973,6 +973,7 @@ async def subscribe(request):
         subtitle_mode=o['subtitle_mode'],
         ytdl_options_presets=o['ytdl_options_presets'],
         ytdl_options_overrides=o['ytdl_options_overrides'],
+        sponsorblock=o['sponsorblock'],
         title_regex=post.get('title_regex'),
         skip_subscriber_only=skip_subscriber_only,
         clip_start=sub_clip_start,