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/components/layout/TopBar.tsx

@@ -1,18 +1,31 @@
+import { Settings } from 'lucide-react'
 import muliLogo from '../../assets/muli-logo.png'
 
+interface TopBarProps {
+  onOpenSettings: () => void
+}
+
 /**
- * Slim top bar — just the logo. The active heap badge moved into the
- * Heaps panel in the left sidebar (where it actually relates to the
- * heap rows the user navigates to).
+ * Slim top bar — logo on the left, settings gear on the right. The
+ * active heap badge moved into the Heaps panel in the left sidebar
+ * (where it actually relates to the heap rows the user navigates to).
  */
-export function TopBar() {
+export function TopBar({ onOpenSettings }: TopBarProps) {
   return (
     <header className="flex h-12 items-center justify-between border-b border-border bg-surface px-4">
       <div className="flex items-center gap-2">
         <img src={muliLogo} alt="Mulimago" className="h-7 w-7 object-contain" />
         <h1 className="text-lg font-semibold text-text">Mulimago</h1>
       </div>
-      <div className="flex items-center gap-2" />
+      <div className="flex items-center gap-2">
+        <button
+          onClick={onOpenSettings}
+          className="rounded p-1.5 text-text-muted transition-colors hover:bg-surface-2 hover:text-text"
+          title="Settings"
+        >
+          <Settings className="h-4 w-4" />
+        </button>
+      </div>
     </header>
   )
 }