bsky: remux HLS video playlists to MP4 via ffmpeg

bsky video embeds expose only an m3u8 playlist, which Telegram cannot
fetch. Download the master/variant playlists and TS segments through the
shared client (proxy-aware, size-capped), then concat-remux locally;
keep the temp dir alive via Fetched._keep_alive like the ugoira path.
Also adds site::download_media_limited (streaming size cap), status
checks on media_size, and moves the ffmpeg probe to site/mod.rs for
pixiv/bsky to share.

crates/x-media/src/site/pixiv/api.rs

@@ -207,8 +207,8 @@ impl PixivAPI {
         &self,
         illust_id: u64,
     ) -> Result<Option<(String, tempfile::TempDir)>, PixivError> {
-        if !ffmpeg_available() {
-            log_once_ffmpeg_missing();
+        if !crate::site::ffmpeg_available() {
+            crate::site::log_once_ffmpeg_missing();
             return Ok(None);
         }
         let metadata = self.ugoira_metadata(illust_id).await?;
@@ -315,28 +315,6 @@ impl PixivAPI {
     }
 }
 
-static FFMPEG_AVAILABLE: LazyLock<bool> = LazyLock::new(|| {
-    std::process::Command::new("ffmpeg")
-        .arg("-version")
-        .stdout(std::process::Stdio::null())
-        .stderr(std::process::Stdio::null())
-        .status()
-        .map(|s| s.success())
-        .unwrap_or(false)
-});
-
-static FFMPEG_MISSING_LOGGED: AtomicBool = AtomicBool::new(false);
-
-fn ffmpeg_available() -> bool {
-    *FFMPEG_AVAILABLE
-}
-
-fn log_once_ffmpeg_missing() {
-    if !FFMPEG_MISSING_LOGGED.swap(true, Ordering::Relaxed) {
-        log::warn!("ffmpeg not found; pixiv ugoira posts stay unsupported");
-    }
-}
-
 /// pixiv3-rs replacement: `None` when `PIXIV_REFRESH_TOKEN` is unset.
 static PIXIV_CLIENT: LazyLock<Option<PixivAPI>> =
     LazyLock::new(|| env::var("PIXIV_REFRESH_TOKEN").ok().map(PixivAPI::new));