Jellyfin: live Ping-only keepalive A/B for transcoded downloads (D3 / B55) #400

Keepalive currently sends fixed-zero interim progress reports. This stops position advancement, but it does not prove watch-state neutrality or encoder lifetime. A live A/B against a real Jellyfin server (Ping-only vs. fixed-zero) is still needed; it is blocked on test-server access.

Deferred from the downloads overhaul (stack #371 → #389, integration branch downloads/overhaul). See the deferred-work section of docs/plans/2026-10-16-downloads-overhaul.md.