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/alembic/versions/060_usd_master_asset_type.py

@@ -0,0 +1,21 @@
+"""Add usd_master to mediaassettype enum.
+
+Revision ID: 060
+Revises: 059
+"""
+from alembic import op
+
+revision = "060"
+down_revision = "059"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TYPE media_asset_type ADD VALUE IF NOT EXISTS 'usd_master'")
+
+
+def downgrade() -> None:
+    # PostgreSQL does not support removing enum values.
+    # The 'usd_master' value will remain but is no longer referenced by application code.
+    pass

backend/alembic/versions/061_material_assignment_layers.py

@@ -0,0 +1,25 @@
+"""Add three-layer material assignment columns to cad_files.
+
+Revision ID: 061
+Revises: 060
+"""
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = "061"
+down_revision = "060"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("cad_files", sa.Column("source_material_assignments", postgresql.JSONB(), nullable=True))
+    op.add_column("cad_files", sa.Column("resolved_material_assignments", postgresql.JSONB(), nullable=True))
+    op.add_column("cad_files", sa.Column("manual_material_overrides", postgresql.JSONB(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("cad_files", "manual_material_overrides")
+    op.drop_column("cad_files", "resolved_material_assignments")
+    op.drop_column("cad_files", "source_material_assignments")

backend/alembic/versions/062_rename_tessellation_settings.py

@@ -0,0 +1,25 @@
+"""Rename gltf_preview/gltf_production tessellation settings keys.
+
+Revision ID: 062
+Revises: 061
+"""
+from alembic import op
+
+revision = "062"
+down_revision = "061"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("UPDATE system_settings SET key = 'scene_linear_deflection'   WHERE key = 'gltf_preview_linear_deflection'")
+    op.execute("UPDATE system_settings SET key = 'scene_angular_deflection'  WHERE key = 'gltf_preview_angular_deflection'")
+    op.execute("UPDATE system_settings SET key = 'render_linear_deflection'  WHERE key = 'gltf_production_linear_deflection'")
+    op.execute("UPDATE system_settings SET key = 'render_angular_deflection' WHERE key = 'gltf_production_angular_deflection'")
+
+
+def downgrade() -> None:
+    op.execute("UPDATE system_settings SET key = 'gltf_preview_linear_deflection'    WHERE key = 'scene_linear_deflection'")
+    op.execute("UPDATE system_settings SET key = 'gltf_preview_angular_deflection'   WHERE key = 'scene_angular_deflection'")
+    op.execute("UPDATE system_settings SET key = 'gltf_production_linear_deflection' WHERE key = 'render_linear_deflection'")
+    op.execute("UPDATE system_settings SET key = 'gltf_production_angular_deflection' WHERE key = 'render_angular_deflection'")