[Enhancement] Run fast indexing on source creation and at higher priority

[kieraneglin]

What's new?

N/A

What's changed?

• Sources now kick off fast indexing immediately when created (if that option is checked)
  • This allows the first handful of media items to get enqueued for download on source creation even if there are long-running indexing tasks going on in the background
• Media downloads queued up by fast indexing have a higher priority than downloads from slow indexing
• Related to [Triage] Restarts break ongoing initial indexings #573 (comment)

What's fixed?

• Updated the count of concurrent fast indexing queues to respect yt_dlp_worker_count since these workers do indeed call out to yt-dlp

Any other comments?

N/A