feat(dl-queue): Add POST /queue/retry endpoint

[Copilot]

Adds an endpoint to retry a specific failed download request by resetting its status to PENDING.

• New handler src/handlers/post-queue-retry.ts: Accepts { id: number }, validates the request exists and is not in PENDING/IN_PROGRESS state, then updates status to PENDING and updated to current date.
• Route registration in src/index.ts: POST /queue/retry

curl -X POST /queue/retry \
  -H "Content-Type: application/json" \
  -H "Authorization: <secret>" \
  -d '{"id": 42}'
# 200 { "id": 42, "status": "PENDING", "updated": "..." }
# 400 if PENDING or IN_PROGRESS
# 404 if not found