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/ClientAssets.h

@@ -37,6 +37,12 @@ namespace ClientAssets {
 	// Where a file inside res/ is on disk (matched ignoring case), if it exists
 	std::optional<std::filesystem::path> ResolveResFile(const std::string& relativePath);
 
+	// The client's res/ folder (empty when client_location isn't set)
+	std::filesystem::path ResFolder();
+
+	// ResolveResFile inside a res/ folder from ResFolder: reads no settings, so worker threads may call it
+	std::optional<std::filesystem::path> ResolveResFile(const std::string& relativePath, const std::filesystem::path& res);
+
 	// The res/-relative path of a file called `fileName` (ignoring case) anywhere under res/<folder>
 	std::optional<std::string> FindResFile(const std::string& folder, const std::string& fileName);