feat: rich product metadata extraction from STEP files

Extract volume, surface area, part count, assembly hierarchy, and
complexity from STEP files via OCC B-rep analysis.

Backend:
- extract_rich_metadata() in step_processor.py: computes per-part volume
  (BRepGProp), surface area, triangle/vertex count, assembly depth,
  instance count, complexity score, largest part identification
- cad_metadata JSONB column on Product model (DB migration)
- Auto-populated during STEP processing (non-fatal, 10s timeout)
- Also stored in cad_files.mesh_attributes["rich_metadata"]
- Batch re-extract endpoint: POST /admin/settings/reextract-rich-metadata

AI Agent:
- search_products returns part_count, volume_cm3, complexity, largest_part
- query_database tool description documents cad_metadata schema

Frontend:
- ProductDetail page: CAD Metadata section with stat cards
  (parts, volume, surface area, complexity, triangles, assembly depth)
- Admin System Tools: "Re-extract Rich Metadata" button for backfill

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/routers/admin.py

@@ -532,6 +532,16 @@ async def reextract_all_metadata(
     return {"queued": queued, "message": f"Queued {queued} CAD file(s) for metadata re-extraction"}
 
 
+@router.post("/settings/reextract-rich-metadata", status_code=status.HTTP_202_ACCEPTED)
+async def reextract_rich_metadata(
+    admin: User = Depends(require_global_admin),
+):
+    """Queue a batch task to re-compute volume, surface area, complexity for all products with STEP files."""
+    from app.tasks.step_tasks import reextract_rich_metadata_task
+    reextract_rich_metadata_task.delay()
+    return {"queued": True, "message": "Rich metadata re-extraction task queued"}
+
+
 @router.post("/settings/generate-missing-canonical-scenes", status_code=status.HTTP_202_ACCEPTED)
 async def generate_missing_canonical_scenes(
     admin: User = Depends(require_global_admin),