feat(sites): add bilibili dynamic support (images and animated images)

Fetch `t.bilibili.com/<id>`, `www.bilibili.com/opus/<id>`,
`t.bilibili.com/h5/dynamic/detail/<id>` and `m.bilibili.com/dynamic/<id>`
through the anonymous `/x/polymer/web-dynamic/v1/detail` endpoint (no
cookie, no WBI signature; the site adds the device cookies
`/x/frontend/finger/spi` hands out, which is what lifts bilibili's
`-352` risk control).

Media: the `major.draw` grid (`.gif` sources become animations, the rest
photos with a downscaled `@518w.jpg` thumbnail used both as preview and
as the oversized fallback), an attached video's cover, and the quoted
dynamic's media for forwards. The video stream itself is not resolved;
`b23.tv` short links stay unmatched (they mostly point at videos, so
matching them would turn a silently ignored link into a failure reply).

`-352`/`-412` map to a retryable error so the queue backs off instead of
dropping the post; a removed dynamic (`500`) is permanent.

Registry-driven, so no bot-side code changes beyond the site lists in the
command replies; found while researching nazurin and
telegram-bili-feed-helper (see BILIBILI_PLAN.md).

crates/xmedia-bot/src/config.rs

@@ -1,5 +1,6 @@
 //! Central env handling. The only other places that read env are
-//! `Bot::from_env` (TELOXIDE_TOKEN) and x-media (PIXIV_REFRESH_TOKEN).
+//! `Bot::from_env` (TELOXIDE_TOKEN) and x-media (PIXIV_REFRESH_TOKEN,
+//! TWITTER_AUTH_TOKEN, BILIBILI_COOKIE).
 
 use std::env;
 use std::net::IpAddr;