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/mod.rs

@@ -313,14 +313,17 @@ static FFMPEG_AVAILABLE: LazyLock<bool> = LazyLock::new(|| {
 
 static FFMPEG_MISSING_LOGGED: AtomicBool = AtomicBool::new(false);
 
-pub(crate) fn ffmpeg_available() -> bool {
-    *FFMPEG_AVAILABLE
-}
-
-pub(crate) fn log_once_ffmpeg_missing() {
+/// Whether the encode step must be skipped: no ffmpeg on PATH, logged once
+/// per process. The one gate both encoders check — the probe and the
+/// log-once used to be two functions that only ever appeared together.
+pub(crate) fn ffmpeg_missing() -> bool {
+    if *FFMPEG_AVAILABLE {
+        return false;
+    }
     if !FFMPEG_MISSING_LOGGED.swap(true, Ordering::Relaxed) {
         log::warn!("ffmpeg not found; ugoira and bsky video posts stay unsupported");
     }
+    true
 }
 
 /// Site adapter: one impl per supported site (twitter / bsky / misskey /