nautd: hash-check paused torrents (check-only worker)

A paused torrent never ran a worker, so it never verified on-disk data
and showed no progress. Add a one-shot check-only swarm mode (open +
resume scan + report, no peers/engine/download). The reconciler runs it
for a paused torrent flagged needs_check (set on paused-add and recheck);
the worker stays paused afterward. New TORRENT_CHECKING state -> webui
checkingDL/UP. Mark #11 done.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

include/naut/swarm.h

@@ -109,6 +109,8 @@ typedef struct {
     uint64_t torrent_id;
     naut_event_bus *events;   /* optional */
     bool keep_alive;          /* retain completed storage until stopped */
+    bool check_only;          /* hash-verify existing data + report, then return;
+                               * no peers, no engine, no download (paused recheck)*/
     naut_swarm_progress_cb on_progress;
     naut_swarm_control_cb on_control;
     naut_swarm_stop_cb should_stop;