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>

tests/integration/run_tracker_swarm.sh

@@ -70,7 +70,8 @@ if [ "$MODE" = "udp" ]; then
 else
   announce="http://127.0.0.1:$tracker_port/announce"
 fi
-python3 - "$SOURCE_TOR" "$tor" "$announce" <<'PY'
+backup="http://127.0.0.1:1/lower-tier-should-not-be-contacted"
+python3 - "$SOURCE_TOR" "$tor" "$announce" "$backup" <<'PY'
 import sys
 
 
@@ -90,7 +91,8 @@ def skip(data, pos):
     return colon + 1 + size
 
 
-source, target, announce = sys.argv[1], sys.argv[2], sys.argv[3].encode()
+source, target = sys.argv[1], sys.argv[2]
+announce, backup = sys.argv[3].encode(), sys.argv[4].encode()
 data = open(source, "rb").read()
 pos = 1
 raw_info = None
@@ -107,6 +109,8 @@ while data[pos] != ord("e"):
 assert raw_info is not None
 rewritten = (
     b"d8:announce" + str(len(announce)).encode() + b":" + announce
+    + b"13:announce-listll" + str(len(announce)).encode() + b":" + announce
+    + b"el" + str(len(backup)).encode() + b":" + backup + b"ee"
     + b"4:info" + raw_info + b"e"
 )
 open(target, "wb").write(rewritten)
@@ -125,5 +129,10 @@ if ! grep -q "REQUEST" "$tracker_log"; then
   echo "FAIL: tracker received no announce"
   exit 1
 fi
+if grep -q "$backup" "$swarm_log"; then
+  echo "FAIL: lower tracker tier was contacted after primary success"
+  cat "$swarm_log"
+  exit 1
+fi
 
 echo "PASS: $MODE tracker discovery produced byte-identical output"