feat: settings panel + thumbnail pipeline fixes

- photos.py: stop crashing in FileResponse when a thumb hasn't been
  generated; return a clean 404 with Retry-After so the frontend can
  back off.
- thumbs.py: fix process_video_thumbnail (overwrite_output, robust
  duration probe across stream/format, eager frame load + temp cleanup)
  so videos stop ending up as the gray placeholder.
- library.py: new /maintenance/* endpoints — thumbnail-stats,
  regenerate-thumbnails (with media_type / only_failed filters), and a
  manual data-integrity cleanup trigger.
- Frontend Settings panel (gear in TopBar) surfacing those endpoints
  plus a re-scan button and live thumbnail status counts.
- PhotoThumbnail: stretch the auto-retry schedule for slow RAW jobs.

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

frontend/src/App.tsx

@@ -10,6 +10,7 @@ import { AppFooter } from './components/AppFooter'
 import { PreviewView } from './components/preview/PreviewView'
 import { FilterBar } from './components/filter/FilterBar'
 import { DiscardActionBar } from './components/discard/DiscardActionBar'
+import { SettingsDialog } from './components/dialogs/SettingsDialog'
 import { usePhotoStore } from './store/photoStore'
 import { useKeyboardShortcuts } from './hooks/useKeyboardShortcuts'
 import { useFilterUrlSync } from './hooks/useFilterUrlSync'
@@ -18,6 +19,7 @@ import { usePhotosQuery } from './hooks/usePhotosQuery'
 function App() {
   const [leftSidebarOpen, setLeftSidebarOpen] = useState(true)
   const [rightSidebarOpen, setRightSidebarOpen] = useState(false)
+  const [settingsOpen, setSettingsOpen] = useState(false)
   const selectedPhotos = usePhotoStore((state) => state.selectedPhotos)
   const viewMode = usePhotoStore((state) => state.viewMode)
 
@@ -53,7 +55,7 @@ function App() {
 
   return (
     <div className="flex flex-col h-screen bg-bg text-text">
-      <TopBar />
+      <TopBar onOpenSettings={() => setSettingsOpen(true)} />
 
       <div className="flex flex-1 overflow-hidden">
         {/* Left Sidebar */}
@@ -100,6 +102,12 @@ function App() {
 
       {/* Preview overlay — covers TopBar when active */}
       {viewMode === 'preview' && <PreviewView />}
+
+      {/* Settings panel — admin/maintenance actions */}
+      <SettingsDialog
+        isOpen={settingsOpen}
+        onClose={() => setSettingsOpen(false)}
+      />
     </div>
   )
 }