docs: explain download quality resolutions and when a preset keeps the original

[Quick104]

Silo-Server/silo-server#1670 pairs each download quality preset with a resolution. Its app PRs, Silo-Server/silo-apple#547 and Silo-Server/silo-android#415, show that resolution in the quality pickers. The site update waits for those PRs to merge.

What changed for users

• Each converted download preset now has a maximum resolution. The iPhone, iPad and Android apps show it in the quality list, for example 10 Mbps · up to 1080p:

  Preset	Up to
  20 Mbps	4K, or 1080p when the server doesn't allow 4K transcoding
  10 Mbps	1080p
  5 Mbps	1080p
  2 Mbps	720p
  1 Mbps	480p, or 540p when HEVC encoding is on

• A converted download is never larger than the original in resolution or bitrate, and stays within what the device can decode. Choosing a preset for a title that already fits can give you the original file instead of a converted copy.

• A title in 4K converts only when Allow 4K transcoding is on in the admin playback settings, the same setting streaming uses. Allow HEVC encoding now also applies to downloads, for devices that can play HEVC.

Pages to update

• src/content/docs/docs/using-silo/downloads.md, step 3 of "Save a movie or episode": "If you picked a smaller quality, the server prepares a copy before the transfer starts" is no longer always true, because a title that already fits the preset downloads as the original. The page could also explain what the "up to" resolution in the quality list means.
• The site doesn't describe Allow 4K transcoding or Allow HEVC encoding today. If a playback settings page is added, it should say both apply to converted downloads as well as streaming.

Out of scope

• Android TV and Apple TV have no downloads.
• The web app's "save the original file" flow is unchanged.

AI disclosure

• Harness: Claude Code, running in T3 Code
• Tool(s): Claude Code
• Model(s): claude-opus-5-5
• Involvement: Fully AI-generated at the maintainer's request.