feat: write playlist/channel metadata files where their items go (#660)

yt-dlp emits the feed-level .info.json, description and thumbnail from
__process_playlist_result without consulting `download`, so they fell out of
MeTube's classification pass with nothing steering them: they landed in
DOWNLOAD_DIR under yt-dlp's own pl_* names, ignoring OUTPUT_TEMPLATE, the
download's folder and AUDIO_DOWNLOAD_DIR. #1040 reported the names; #660 asked
for control over them. Same accident from both sides.

The classification pass now writes nothing at all, and the files are produced
once the feed has been accepted and its type is known, reusing the template its
items use — OUTPUT_TEMPLATE_CHANNEL for a channel, OUTPUT_TEMPLATE_PLAYLIST for
a playlist, falling back to OUTPUT_TEMPLATE — evaluated against the feed dict.
With the defaults that puts them in the same folder as the videos, named after
the feed, which is the Jellyfin layout #660 asked for. No new environment
variable: knowing the feed type is what makes reusing the item template
possible, and doing this after extraction is what makes the type known.

The write re-runs yt-dlp over a copy of the feed with its entries removed,
which reaches the playlist-file writing without re-extracting anything and
without touching yt-dlp's private write helpers. It runs in an executor and
never fails the add.

Nothing new appears for anyone who hasn't enabled writeinfojson /
writethumbnail, an explicit allow_playlist_files=false still turns it off, and
a failed or cancelled add no longer leaves metadata behind.

Subscription scans keep allow_playlist_files=False: a scan is a timer-driven
poll, and items it finds are queued through the download queue, which does the
writing. Previously every check interval rewrote these files.

Test doubles for __extract_info now take *args/**kwargs.

README.md

@@ -83,6 +83,8 @@ Certain values can be set via environment variables, using the `-e` parameter on
 * __ALLOW_PRIVATE_ADDRESSES__: Whether to allow downloads from private, loopback, link-local and other non-global addresses. Defaults to `false`, which protects against SSRF by refusing URLs that resolve to internal hosts. Set to `true` only in trusted environments — for example when routing traffic through a proxy/VPN client in Fake-IP mode (sing-box, Clash, Mihomo), which resolves hosts to the `198.18.0.0/15` range. Enabling this disables the SSRF protection entirely, so only use it when you control the network.
 * __YTDL_NIGHTLY_UPDATE_TIME__: If set, will cause MeTube to use [nightly yt-dlp builds](https://github.com/yt-dlp/yt-dlp-nightly-builds) instead of the stable releases. Set to the time (`HH:MM`, 24-hour) when you want the daily upgrades and MeTube restart to happen. Defaults to empty (disabled).
 
+Enabling `writeinfojson` or `writethumbnail` in `YTDL_OPTIONS` also writes a feed-level `.info.json` and thumbnail when you add a playlist or channel. These reuse the template of the items they belong to — `OUTPUT_TEMPLATE_CHANNEL` or `OUTPUT_TEMPLATE_PLAYLIST` — evaluated against the feed itself, so with the defaults they land in the same folder as the videos, named after the feed. Set `allow_playlist_files` to `false` in `YTDL_OPTIONS` to skip them.
+
 ### 🌐 Web Server & URLs
 
 * __HOST__: The host address the web server will bind to. Defaults to `0.0.0.0` (all interfaces).