feat: layout hamburger, media browser filters+previews, billing fixes

- Layout: mobile hamburger menu + overlay backdrop + close button; content area always full-width
- Media browser: filter chips (default still+turntable); advanced toggle for GLB/STL; thumbnail_url previews for non-image types; video hover-play for turntable
- Backend: asset_types multi-filter, thumbnail_url in MediaAssetOut, download proxy endpoint for MinIO/local files
- Admin: "Import Existing Media" button → POST /api/admin/import-media-assets
- Billing: fix invoice create 500 (MissingGreenlet — use selectinload after commit); PDF download uses axios blob instead of bare <a href> (auth header missing); fix storage.upload() accepting str|Path
- SSE task logs: task_logs.py core + router, LiveRenderLog component
- CadPreview: fix infinite loop when no gltf_geometry assets; loading screen before ThreeDViewer render
- render-worker: add trimesh layer to Dockerfile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Admin.tsx

@@ -141,6 +141,14 @@ export default function AdminPage() {
     onError: (e: any) => toast.error(e.response?.data?.detail || 'Failed'),
   })
 
+  const importMediaAssetsMut = useMutation({
+    mutationFn: () => api.post('/admin/import-media-assets'),
+    onSuccess: (res) => {
+      toast.success(`Imported: ${res.data.created} created, ${res.data.skipped} skipped`)
+    },
+    onError: (e: any) => toast.error(e.response?.data?.detail || 'Import failed'),
+  })
+
   const generateMissingStlsMut = useMutation({
     mutationFn: () => api.post('/admin/settings/generate-missing-stls'),
     onSuccess: (res) => {
@@ -666,6 +674,18 @@ export default function AdminPage() {
                 </button>
                 <p className="text-xs text-content-muted">Re-renders thumbnails for all completed CAD files.</p>
               </div>
+              <div className="flex flex-col gap-1">
+                <button
+                  onClick={() => importMediaAssetsMut.mutate()}
+                  disabled={importMediaAssetsMut.isPending}
+                  className="btn-secondary text-sm w-full justify-start"
+                  title="Create MediaAsset records for all existing CAD thumbnails and order line renders"
+                >
+                  <RefreshCw size={14} className={importMediaAssetsMut.isPending ? 'animate-spin' : ''} />
+                  {importMediaAssetsMut.isPending ? 'Importing…' : 'Import Existing Media'}
+                </button>
+                <p className="text-xs text-content-muted">Registers existing renders &amp; CAD thumbnails in the Media Browser.</p>
+              </div>
               <div className="flex flex-col gap-1">
                 <button
                   onClick={() => generateMissingStlsMut.mutate()}