perf: speed up settings dialog + relocate settings entry point

- Parallelize the six celery inspect.*() calls in /library/maintenance/
  worker-status via asyncio.gather + to_thread, and drop per-call
  timeout from 1.0s to 0.5s. Endpoint goes from ~6.1s to ~0.54s — it
  was the sole bottleneck on opening the Settings dialog.
- SettingsDialog now fetches through React Query with enabled:isOpen,
  so reopening shows cached data instantly while a background refetch
  updates. Worker polling moved to refetchInterval. Loading spinners
  only show when there's no cached data yet, so background refetches
  don't keep them spinning.
- Move the Settings entry point from the TopBar to a pinned row at the
  bottom of the LeftSidebar so it sits alongside the other library
  controls. TopBar no longer takes onOpenSettings.
- Remove the "Scan all folders" bottom action from LeftSidebar — the
  same control already lives in Settings → Library → Re-scan.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -44,7 +44,6 @@ function App() {
   return (
     <div className="flex flex-col h-screen bg-bg text-text">
       <TopBar
-        onOpenSettings={() => setSettingsOpen(true)}
         leftSidebarOpen={leftSidebarOpen}
         rightSidebarOpen={showRightSidebar}
         onExpandLeft={() => setLeftSidebarOpen(true)}
@@ -58,7 +57,10 @@ function App() {
             leftSidebarOpen ? 'w-64' : 'w-0'
           } overflow-hidden border-r border-border bg-surface`}
         >
-          <LeftSidebar onCollapse={() => setLeftSidebarOpen(false)} />
+          <LeftSidebar
+            onCollapse={() => setLeftSidebarOpen(false)}
+            onOpenSettings={() => setSettingsOpen(true)}
+          />
         </div>
 
         {/* Main column — filter bar, discard bar, timeline. Lives to the