feat(P2): USD Foundation — canonical part identity + material overrides

M1 — USD exporter:
- render-worker/scripts/export_step_to_usd.py (631 lines)
  Full XCAF traversal, one UsdGeom.Mesh per leaf part,
  schaeffler:partKey on every prim, index-space sharpEdgeVertexPairs
- render-worker/Dockerfile: usd-core>=24.11 installed (USD 0.26.3)

M2 — usd_master MediaAsset + pipeline auto-chain:
- migrations 060 (usd_master enum), 061 (3 JSONB columns),
  062 (rename tessellation settings keys)
- generate_usd_master_task: runs export_step_to_usd.py, upserts
  usd_master MediaAsset, writes resolved_material_assignments to CadFile
- Auto-chained from generate_gltf_geometry_task after every GLB export
- step_tasks.py shim re-exports generate_usd_master_task

M3 — scene-manifest API:
- part_key_service.py: build_scene_manifest(), generate_part_key(),
  four-layer material priority resolution with provenance
- SceneManifest / PartEntry Pydantic models in products/schemas.py
- GET /api/cad/{id}/scene-manifest endpoint (graceful fallback to
  parsed_objects when USD not yet generated)
- POST /api/cad/{id}/generate-usd-master endpoint
- frontend/src/api/sceneManifest.ts: fetchSceneManifest(),
  triggerUsdMasterGeneration()

M4 — manual-material-overrides API:
- GET/PUT /api/cad/{id}/manual-material-overrides endpoints
- CadFile.manual_material_overrides JSONB column (migration 061)
- getManualOverrides() / saveManualOverrides() in cad.ts

M5 — ThreeDViewer partKey integration:
- export_step_to_gltf.py injects partKeyMap into GLB extras
- ThreeDViewer: partKeyMap extraction, resolvePartKey(), effectiveMaterials
  merges legacy partMaterials + new manualOverrides (server-side persistence)
- MaterialPanel: dual-path save (partKey vs legacy), provenance badge,
  reconciliation panel for unmatched/unassigned parts

Also:
- Admin.tsx: generate-missing-usd-masters + canonical scenes bulk actions
- ProductDetail.tsx: usd_master row in asset table
- vite-env.d.ts: fix ImportMeta.env TypeScript error
- GPUProbeResult: add timestamp/devices/render_time_s fields

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

frontend/src/api/cad.ts

@@ -151,3 +151,32 @@ export async function savePartMaterials(
   const res = await api.put<PartMaterialsResponse>(`/cad/${cadFileId}/part-materials`, map)
   return res.data.part_materials ?? {}
 }
+
+// ---------------------------------------------------------------------------
+// Manual material overrides (partKey-keyed, Priority 4)
+// ---------------------------------------------------------------------------
+
+export interface ManualMaterialOverridesResponse {
+  cad_file_id: string
+  manual_material_overrides: Record<string, string> | null
+}
+
+/** Return the manual material overrides for a CAD file ({partKey: materialName}, empty if none). */
+export async function getManualOverrides(cadFileId: string): Promise<Record<string, string>> {
+  const res = await api.get<ManualMaterialOverridesResponse>(
+    `/cad/${cadFileId}/manual-material-overrides`,
+  )
+  return res.data.manual_material_overrides ?? {}
+}
+
+/** Save manual material overrides keyed by partKey. Returns the saved map. */
+export async function saveManualOverrides(
+  cadFileId: string,
+  overrides: Record<string, string>,
+): Promise<Record<string, string>> {
+  const res = await api.put<ManualMaterialOverridesResponse>(
+    `/cad/${cadFileId}/manual-material-overrides`,
+    { overrides },
+  )
+  return res.data.manual_material_overrides ?? {}
+}