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>

LEARNINGS.md

@@ -454,3 +454,6 @@ In OCP (pybind11-basiert) gibt jeder Aufruf von `solid.TShape()` ein neues Pytho
 
 ### 2026-03-11 | GMSH | CharacteristicLengthMax vs. OCC linear_deflection
 OCC `linear_deflection=0.1mm` auf einem 50mm-Zylinder → Kantenlänge ~5mm. GMSH `CharacteristicLengthMax=0.1×15=1.5mm` → 3× mehr Unterteilungen → 9× mehr Dreiecke → GLB 7× größer. **Lösung:** `CharacteristicLengthMax = linear_deflection × 50` (≈5mm), `MinimumCirclePoints = min(12, ...)` statt min(20). Ergebnis: GMSH 91% von OCC-Größe (Ziel ≤120% ✓).
+
+### 2026-03-12 | GMSH | Priority 3 vollständig — GMSH-Pipeline Status
+GMSH 4.15.1 in render-worker installiert. `tessellation_engine=gmsh` ist der aktive DB-Default. `_tessellate_with_gmsh()` in `export_step_to_gltf.py` vollständig: `CharacteristicLengthMax = linear_deflection × 50`, `MinimumCirclePoints = min(12, ...)`, REVERSED Solids bleiben erhalten (kein invertierter Jacobian). Produktion-GLB nutzt Cache-Reuse (kein Re-Tessellieren bei Materialwechsel). Sharp-Edge-Extraktion läuft nach Tessellierung unabhängig vom Engine-Typ — `Injected N segment pairs into GLB extras` gilt für beide Pfade.