The agent ignored its config.toml method preference for web-driven downloads
(only the local `unarr download` command read it), and resolveMethod tried
torrent first in auto mode — so a 'debrid only' user still got torrent tasks.
- config: preferred_methods (ordered list, e.g. ["debrid","usenet"]) with
MethodOrder() resolution; back-compat with the singular preferred_method.
Methods absent from the list are disabled (debrid-only never torrents).
- resolveMethod/tryFallback honor the config order (gating, no fallback to a
method outside the list) over the per-task preference.
- report preferred_methods on register so the web honors it (resolves debrid,
gates the P2P stream fallback).
- enable the usenet downloader when usenet is listed (it was never enabled).
- config_menu: ordered presets (debrid-only, debrid→torrent, debrid→usenet…).
Tests: resolveMethod gating + fallback within/outside the list.
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