perf: stop the sweep stealing worker wakeups, retry-free inline fetch

- queue: the lease-expiry sweep waited on the workers' `Notify`. `notify_one`
  stores a permit, so a sweep wakeup could consume the one meant for a worker,
  which then blocked on `notified()` (it only waits when the table looked
  empty, i.e. indefinitely) with a due row sitting there. The sweep now has
  its own notify, woken only by stop.
- x-media: split fetch's retry loop into `fetch` (3 attempts, unchanged) and
  `fetch_once` (1 attempt); inline queries use the latter — the 800ms debounce
  plus 1s/2s backoffs were outlasting the answer window of the query.
- send: chunk_media_items now moves items out of the input Vec instead of
  requiring `T: Clone` and copying every payload.

fmt/clippy clean, 55 + 69 tests pass.

crates/xmedia-bot/src/handlers/inline.rs

@@ -121,7 +121,9 @@ async fn answer_inline_query(bot: Bot, query: InlineQuery) -> Result<bool, Reque
         query.query,
         log_key(&query.query)
     );
-    match x_media::site::fetch(&query.query).await {
+    // No retries: the debounce plus a 1s/2s backoff would outlast the inline
+    // query the answer belongs to.
+    match x_media::site::fetch_once(&query.query).await {
         Ok(Some(fetched)) => {
             let mut results: Vec<InlineQueryResult> = Vec::new();
             // Inline results have the same 1024-char caption limit as regular