Downloads: only one transcode/convert job is sent to the server at a time, so queued batches don't render while the app is suspended #414

Problem

With the downloads overhaul (P4, #385), Plex optimize / Emby Convert jobs go through a server-preparation admission lane capped at 1 (DownloadAdmissionLimits: .serverPreparation: 1, regardless of the simultaneous-downloads setting).

The handoff from "render finished" to "start the byte transfer" is app-side polling, which cannot run while the app is suspended (documented in docs/DOWNLOADS-OFFLINE.md). With the cap at 1, the only job the server knows about is the first one:

1. The user queues three transcoded downloads and locks the phone / puts it away overnight.
2. Only job 1 is sent to the server. It renders, but the app is suspended, so nothing notices and nothing downloads.
3. Jobs 2 and 3 stay as Waiting for a download slot… and are never sent to the server.
4. On opening the app, the poller sees job 1 ready, hands it to the static lane, and only then sends job 2. Job 3 waits again for job 2's render to be seen while the app is awake.

Net effect: a batch queued before bed gets at most one item prepared overnight. Reported on build 1537 (iOS); the cap is cross-platform.

Before the overhaul, single taps submitted their server job right away, so this is a regression in practice.

Expected

Queuing N transcoded downloads submits all N jobs to the server straight away, so the server can render them while the app is suspended. The server's own transcoder settings (for example Plex's background-transcode limit) decide how many render at once. When the app next runs, every finished render hands off to the static lane, subject to the static/total caps.

Proposed direction

• Stop capping server submission at 1. Either remove .serverPreparation from the total/lane window, or give it a separate, larger submission cap (for example the total setting, or a fixed bound such as 5–10) that does not count against the transfer slots.
• Keep the static-lane and total caps for byte transfers. A finished render that finds the static lane full waits as prepared, not as unsubmitted.
• Pollers for every submitted job run whenever the app is awake, including on background-session wakes.
• Pause, delete and sign-out keep cancelling each row's server job (D2), now for several rows at once.
• Emby Convert: confirm that several concurrent Convert jobs are accepted, or keep a lower Emby-specific bound.

Acceptance

• Unit: queuing 3 Plex optimize rows with total = 3 submits 3 server jobs without waiting for a handoff.
• Unit: renders that finish while the static lane is full wait without resubmitting, and start in request order.
• Unit: delete or pause of a submitted-but-unrendered row cancels its server job.
• Docs: DOWNLOADS-OFFLINE.md admission section updated.
• Device (user): queue 3 transcoded downloads, background for several hours, reopen; all 3 renders are complete server-side and the transfers start.