refactor(site): one ffmpeg gate instead of a probe plus a log call

ffmpeg_available() and log_once_ffmpeg_missing() only ever appeared together (both encoders, three lines each: check, log, return), and calling one without the other was a bug in waiting — a probe that never logged, or a log that never gated. ffmpeg_missing() is the single gate: false when the binary is there, otherwise log once per process and say yes. Both call sites shrink to one condition.

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

@@ -206,8 +206,7 @@ impl PixivAPI {
         &self,
         illust_id: u64,
     ) -> Result<Option<(String, tempfile::TempDir)>, PixivError> {
-        if !crate::site::ffmpeg_available() {
-            crate::site::log_once_ffmpeg_missing();
+        if crate::site::ffmpeg_missing() {
             return Ok(None);
         }
         let metadata = self.ugoira_metadata(illust_id).await?;