fix(send): keep local media (ugoira/bsky remux MP4) alive across queue retries

A task whose media is a locally produced file (pixiv ugoira MP4, bsky HLS
remux MP4) references a path inside a tempfile TempDir owned by Fetched.
The retry ran after that TempDir was dropped, so the file was already gone
and the retry always failed permanently ("local media file missing") — and
the upload fallback even tried to GET the local path as a URL.

Keep the temp dirs in a process-wide registry (Fetched::take_keep_alive ->
send::KEEP_ALIVE) that is only released when the task settles (sent or
permanently failed); the upload fallback now uploads local files directly
instead of attempting to download them.

Restart-mid-queue still loses the files (documented behavior in
input_file_for) — only the in-process retry path is fixed here.

crates/x-media/src/site/mod.rs

@@ -95,6 +95,14 @@ impl Fetched {
             )
         })
     }
+
+    /// Hands over the temp dir keeping locally produced media (ugoira MP4,
+    /// bsky remux MP4) alive. The bot keeps it while its task may still be
+    /// retried by the queue, which runs after this [`Fetched`] is dropped and
+    /// its temp files would otherwise be gone. `None` when no such dir exists.
+    pub fn take_keep_alive(&mut self) -> Option<tempfile::TempDir> {
+        self._keep_alive.take()
+    }
 }
 
 /// Renders a user-supplied caption format from raw (already-escaped) field