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/CadPreview.tsx

@@ -1,24 +1,30 @@
+import { useState, useEffect } from 'react'
 import { useParams, useNavigate } from 'react-router-dom'
-import { useQuery } from '@tanstack/react-query'
+import { useQuery, useMutation } from '@tanstack/react-query'
+import { Box, Loader2, X } from 'lucide-react'
 import ThreeDViewer from '../components/cad/ThreeDViewer'
 import { getMediaAssets } from '../api/media'
+import { generateGltfGeometry } from '../api/cad'
 
 /**
  * Route: /cad/:id
  *
  * Full-screen 3D viewer for a CAD file.
- * Passes production GLB URL if a gltf_geometry MediaAsset exists for this CAD file.
+ * If no geometry GLB exists yet, offers to generate one on demand.
  */
 export default function CadPreviewPage() {
   const { id } = useParams<{ id: string }>()
   const navigate = useNavigate()
+  const [generating, setGenerating] = useState(false)
 
   // Load any geometry GLB that was generated for this CAD file
-  const { data: gltfAssets } = useQuery({
+  // Poll every 3s while generating so it appears automatically
+  const { data: gltfAssets, isLoading: gltfLoading } = useQuery({
     queryKey: ['media-assets', id, 'gltf_geometry'],
     queryFn: () => getMediaAssets({ cad_file_id: id!, asset_type: 'gltf_geometry' }),
     enabled: !!id,
-    staleTime: 30_000,
+    staleTime: 5_000,
+    refetchInterval: generating ? 3_000 : false,
   })
 
   // Load production GLB if available
@@ -37,6 +43,20 @@ export default function CadPreviewPage() {
     staleTime: 30_000,
   })
 
+  const generateMutation = useMutation({
+    mutationFn: () => generateGltfGeometry(id!),
+    onSuccess: () => {
+      setGenerating(true)
+    },
+  })
+
+  // Stop polling once asset appears
+  useEffect(() => {
+    if (generating && gltfAssets && gltfAssets.length > 0) {
+      setGenerating(false)
+    }
+  }, [generating, gltfAssets])
+
   if (!id) {
     return (
       <div className="flex items-center justify-center h-full text-content-muted p-8">
@@ -49,6 +69,61 @@ export default function CadPreviewPage() {
   const latestProduction = productionAssets?.[0]
   const latestBlend = blendAssets?.[0]
 
+  // While checking for assets, show a neutral loading screen (don't attempt to render ThreeDViewer)
+  if (gltfLoading) {
+    return (
+      <div className="fixed inset-0 z-50 flex flex-col items-center justify-center bg-gray-950 gap-3">
+        <Loader2 size={36} className="animate-spin text-gray-400" />
+        <p className="text-gray-400 text-sm">Checking for 3D model…</p>
+      </div>
+    )
+  }
+
+  // No GLB available yet — show generate prompt
+  if (!latestGltf) {
+    return (
+      <div className="fixed inset-0 z-50 flex flex-col bg-gray-950">
+        <div className="flex items-center justify-between px-5 py-3 bg-gray-900 border-b border-gray-800">
+          <span className="text-white font-semibold tracking-wide">3D Viewer</span>
+          <button
+            onClick={() => navigate(-1)}
+            className="p-1.5 rounded-md text-gray-400 hover:text-white hover:bg-gray-700 transition-colors"
+          >
+            <X size={20} />
+          </button>
+        </div>
+        <div className="flex-1 flex flex-col items-center justify-center gap-4 text-center px-8">
+          <Box size={48} className="text-gray-600" />
+          <p className="text-white text-lg font-semibold">No 3D model available yet</p>
+          <p className="text-gray-400 text-sm max-w-sm">
+            Generate a GLB file from the STEP cache to enable the 3D viewer.
+            The STL cache must exist (process the STEP file first).
+          </p>
+          {generating ? (
+            <div className="flex items-center gap-2 text-gray-300 text-sm">
+              <Loader2 size={16} className="animate-spin" />
+              Generating… checking every 3s
+            </div>
+          ) : (
+            <button
+              onClick={() => generateMutation.mutate()}
+              disabled={generateMutation.isPending}
+              className="px-5 py-2 rounded-md bg-accent hover:bg-accent-hover disabled:opacity-40 disabled:cursor-not-allowed text-white text-sm font-medium transition-colors flex items-center gap-2"
+            >
+              {generateMutation.isPending && <Loader2 size={14} className="animate-spin" />}
+              Generate 3D Model
+            </button>
+          )}
+          {generateMutation.isError && (
+            <p className="text-red-400 text-sm">
+              Failed to start generation. Check that the STL cache exists.
+            </p>
+          )}
+        </div>
+      </div>
+    )
+  }
+
   return (
     <ThreeDViewer
       cadFileId={id}