feat(inline): answer from the link cache, and put the answer on the trait

Inline results were URL-only: Telegram fetches an inline result's URL itself
and cannot send site headers, so every pixiv item (and every locally encoded
ugoira/bsky MP4) was skipped and such a query answered empty. A post that is
already in the link cache now answers with InlineQueryResultCached* built
from its Telegram file ids — no fetch, no upload, and the hotlink-protected
case simply works. A degraded entry (file ids gone) falls back to URLs, and
there a video with no poster is skipped (Telegram would try to render the
mp4 as its own thumbnail).

The answer call moves onto MediaSender (answer_inline_query, mirroring the
other user-flow methods), which is what makes the path testable at all: the
two new tests drive the cache answer and the degraded/empty answer through
TestStores + MockSender, which recorded nothing about inline before.

The result builders are shared by both paths now (url_result/cached_result +
inline_kind), so the fetch path's behaviour is unchanged.

crates/xmedia-bot/src/link_cache.rs

@@ -15,7 +15,7 @@ use serde::{Deserialize, Serialize};
 use std::sync::Arc;
 use std::time::Duration;
 
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum CachedMediaKind {
     Photo,