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>

backend/app/domains/products/models.py

@@ -32,6 +32,9 @@ class CadFile(Base):
     render_log: Mapped[dict] = mapped_column(JSONB, nullable=True)
     mesh_attributes: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
     part_materials: Mapped[dict | None] = mapped_column(JSONB, nullable=True, default=None)
+    source_material_assignments: Mapped[dict | None] = mapped_column(JSONB, nullable=True, default=None)
+    resolved_material_assignments: Mapped[dict | None] = mapped_column(JSONB, nullable=True, default=None)
+    manual_material_overrides: Mapped[dict | None] = mapped_column(JSONB, nullable=True, default=None)
     step_file_hash: Mapped[str | None] = mapped_column(String(64), nullable=True, index=True)
     tenant_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("tenants.id"), nullable=True, index=True