perf: retry the bsky HLS request, not the whole fetch

A failed bsky video remux was reported as `FetchError::Transient`, so the
fetch loop retried the *whole* adapter — master playlist, variant playlist
and up to 500 segments again (256 MiB of cap each time), for a failure that
happened near the end of the work the retry was about to redo. The message
had to stay honest (returning `Ok` with no media reads as "this post has no
media"), so it needed a class of its own.

`FetchError::MediaPrep` is that class: post fetched, media could not be
prepared locally, not retryable (the per-site `is_retryable` whitelist
excludes it by construction), with its own user-facing text — the generic
"failed to fetch" would have hidden that the download or encode was what
broke. The retry itself is not lost, it moved: `fetch_hls` retries the
request that actually failed, once, for the classes a retry can change
(transport, 429/5xx).

Checked the sibling sites before widening the change: pixiv already degrades
to no media when the ugoira encode fails (`Ok(None)`), and its frame-zip
download failure is the last step so a re-fetch re-does only that; twitter's
auth leg replays two cheap metadata GETs and a GraphQL 5xx *is* worth
retrying. Neither needed the new class.

Verified with a throwaway proxy harness: a 503 answered twice-in-a-row path
costs 2 requests and succeeds, a 404 costs exactly 1 and fails (no pointless
retry). `site::bsky::interface::tests::media_prep_failure_is_not_retried`
pins the classification.

`cargo fmt --check`, `cargo clippy --workspace --all-targets --locked -- -D
warnings` and `cargo test --workspace --locked` clean.

crates/xmedia-bot/src/handlers/urls.rs

@@ -459,6 +459,11 @@ fn fetch_error_message(err: &x_media::site::FetchError) -> String {
         FetchError::Transient(_) | FetchError::Http(_) => {
             "The source site is unavailable right now (tried 3 times). Try again later.".to_string()
         }
+        FetchError::MediaPrep(_) => concat!(
+            "Could not prepare this post's media (its download or encode failed). ",
+            "Try again later."
+        )
+        .to_string(),
         // Parse/shape surprises, pixiv auth details, oversized media: nothing
         // actionable for the user beyond "this did not work".
         _ => "Failed to fetch media from this link.".to_string(),