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/bsky/interface.rs

@@ -153,8 +153,7 @@ async fn fetch_hls(url: &str, cap: u64) -> Result<bytes::Bytes, String> {
 async fn resolve_bsky_video(
     playlist_url: &str,
 ) -> Result<Option<(std::path::PathBuf, tempfile::TempDir)>, String> {
-    if !crate::site::ffmpeg_available() {
-        crate::site::log_once_ffmpeg_missing();
+    if crate::site::ffmpeg_missing() {
         return Ok(None);
     }
     let master = fetch_hls(playlist_url, 1_048_576)