fix: switch BackgroundJobRunner from Vec to FuturesUnordered

BackgroundJobRunner stored active jobs in a Vec<BoxFuture> and polled
ALL of them on every wakeup — O(n) per poll. Since this runs in the
same tokio::select! as the WebServer accept loop, polling overhead from
active connections directly delayed acceptance of new connections.

FuturesUnordered only polls woken futures — O(woken) instead of O(n).

TODO.md

@@ -171,17 +171,6 @@ Pending tasks for AI agents. Remove items when completed.
   | `sdk/base/lib/interfaces/Host.ts`    | SDK `MultiHost.bindPort()` — no changes needed                                                                                                          |
   | `core/src/db/model/public.rs`        | Public DB model — port forward mapping                                                                                                                  |
 
-- [ ] Switch `BackgroundJobRunner` from `Vec<BoxFuture>` to `FuturesUnordered` - @dr-bonez
-
-  **Problem**: `BackgroundJobRunner` (in `core/src/util/actor/background.rs`) stores active jobs in a
-  `Vec<BoxFuture>` and polls ALL of them on every wakeup — O(n) per poll. This runs inside the same
-  `tokio::select!` as the WebServer accept loop (`core/src/net/web_server.rs:502`), so polling overhead
-  from active connections directly delays acceptance of new connections.
-
-  **Fix**: Replace `jobs: Vec<BoxFuture<'static, ()>>` with `jobs: FuturesUnordered<BoxFuture<'static, ()>>`.
-  `FuturesUnordered` only polls woken futures — O(woken) per poll instead of O(n). The poll loop changes
-  from `iter_mut().filter_map(poll_unpin)` + `swap_remove` to `while poll_next_unpin().is_ready() {}`.
-
 - [ ] Extract TS-exported types into a lightweight sub-crate for fast binding generation
 
   **Problem**: `make ts-bindings` compiles the entire `start-os` crate (with all dependencies: tokio,