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>

render-worker/Dockerfile

@@ -61,6 +61,9 @@ RUN pip3 install --no-cache-dir -e .
 # Install cadquery (heavy — installed after backend deps for better layer caching)
 RUN pip3 install --no-cache-dir "cadquery>=2.4.0"
 
+# Install trimesh for STL→GLB geometry export (separate layer to avoid cache invalidation)
+RUN pip3 install --no-cache-dir "trimesh>=4.2.0"
+
 # Copy render scripts
 COPY render-worker/scripts/ /render-scripts/