feat: convert 3D scenery models on worker threads with deferred replies

The dashboard's web server answers one request at a time, so converting a big
.nif (glom files up to tens of MB) held up every other request, flairs included.

- dWeb: Web::Defer hands a request to another thread; the reply is sent from the
  web thread on its next poll (DeferredQueue). A client that leaves first cancels
  it and the late reply is dropped. The synchronous route API is unchanged.
- Web::Shutdown closes connections while the state their close events touch is
  still alive; the destructor no longer runs handlers during static destruction
  (stopping the dashboard aborted in ~WSClient).
- WorkerPool: priority lanes, with one thread only for urgent work (flairs,
  small models, textures), and limited background work.
- Scenery: mesh and texture routes (and the showcase's) convert on the pool;
  thread-safe memory and disk caches, one conversion per model at a time with
  waiters sharing it; zones are converted ahead onto the disk cache while viewed.
- Setting scenery_workers (0: half the cores, 2 to 4).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dDashboardServer/routes/SettingsCatalog.cpp

@@ -278,6 +278,9 @@ namespace {
 		c.Add(Bool(DASHBOARD, "secure_cookies", "HTTPS only cookies", "Turn on when the dashboard is served over HTTPS.", false));
 		c.Add(Bool(DASHBOARD, "behind_proxy", "Behind a reverse proxy", "Use the proxy's X-Forwarded-For for rate limits. Only when the dashboard can't be reached directly.", false));
 		c.Add(Unit(Int(DASHBOARD, "broadcast_interval", "Live update interval", "How often server status is pushed to open pages.", "2000", 250, 60000, true), "ms"));
+		c.Add(Unit(Int(DASHBOARD, "scenery_workers", "3D model conversion threads",
+			"Threads converting the client's models for the 3D views, so the dashboard keeps answering meanwhile. One of them only takes flairs and small models. 0 picks half the CPU cores (2 to 4).",
+			"0", 0, 16, true), "threads"));
 
 		c.AddSection("Keys", "Only in the .ini file or the environment.");
 		c.Add(Secret(DASHBOARD, "jwt_secret", "Session signing secret", "At least 32 characters; empty makes one. Changing it signs everyone out.", true));