webui: replace nautctl web server with a loadable plugin

Drop the web UI that was compiled into nautctl and serve the
torrent-ui front end (../torrent-ui/public) from a native plugin
(plugins/webui) loaded via `nautd --plugin`. The plugin talks to the
engine only through the host call_rpc ABI and adapts the daemon's RPC
surface to the qBittorrent-style contract the UI expects (snapshot/SSE,
torrent detail tabs, add/delete, cookie auth).

Also folds in the daemon refactor that owns per-torrent worker threads
and the swarm engine (naut_swarm) used by the plugin's data source.

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

include/naut/metainfo.h

@@ -35,7 +35,9 @@ typedef struct naut_metainfo {
     const uint8_t *piece_hashes;
 
     naut_file *files;     size_t num_files;
-    char     **trackers;  size_t num_trackers;   /* announce + announce-list, flattened */
+    char     **trackers;  size_t num_trackers;
+    /* Parallel to trackers. Equal values belong to one BEP-12 tier. */
+    uint32_t  *tracker_tiers;
 
     /* internals kept alive so piece_hashes/name stay valid */
     void *_owned;