ditch the per-run batch cap (a 200-video backlog would've taken weeks). now the
'Download YouTube Wishlist' automation queues the ENTIRE wishlist as 'queued'
rows, starts up to max_concurrent (default 3) right away, and each finished
download starts the next (one-out-one-in in the worker) so it all drains in a
controlled stream. the knob is 'max simultaneous downloads', not a total cap.
mirrors the music download worker's lesson (cap concurrency + space starts to
avoid yt-dlp 429s) but stays isolated on the video side:
- youtube_download: _pace() staggers fetch starts (3s); start_next_queued()
claims+spawns the next; run_youtube_download chains on finish.
- db.count_active_youtube_downloads() + claim_next_youtube_queued() (atomic,
race-safe). block field batch_size -> max_concurrent.
all seam-tested (pure select + pump); 680-test sweep green.
7a90b008f0