GET /api/v1/downloads previously serialized every entry in the
in-memory download_tasks dict on every call. With a long-running
server and many historical downloads this produces an unbounded
response payload.
The endpoint now accepts:
limit - max items to return (default 100, clamped to 1..500)
offset - skip first N items (default 0)
status - comma-separated statuses to include (e.g. downloading,queued)
The response now includes total (post-filter count), limit, and
offset so clients can paginate without loading everything first.
Tasks are sorted by status_change_time descending so the newest
activity is on page 1.
Backward compatibility: clients that ignore the new query params
get the same shape plus the extra top-level fields; the downloads
list itself is just capped at 100 instead of unbounded.
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