fix: render pipeline + multi-tenancy bugs (B-Fix-1 through B-Fix-9)

- Remove worker-thumbnail (no Blender, was competing on thumbnail_rendering)
- Move render_order_line_task to thumbnail_rendering queue (render-worker)
- Restore template_service.py real implementation (fix circular import shim)
- Thread tenant_id through STEP upload, Excel import, product create
- Make system tables (output_types, materials, etc.) tenant_id nullable
- Fix tenants frontend 307-redirect: use trailing slash /tenants/
- Remove Flamenco + Three.js from Admin UI (unsupported)
- Set all output_types render_backend to celery (was flamenco)

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

backend/app/services/excel_import.py

@@ -36,6 +36,7 @@ async def import_excel_to_products(
     parsed_rows: list[dict],
     source_excel: str,
     category_key: str | None = None,
+    tenant_id=None,
 ) -> ImportResult:
     """For each row, look up or create a Product.
 
@@ -78,7 +79,7 @@ async def import_excel_to_products(
             "source_excel": source_excel,
         }
 
-        product, was_created = await lookup_or_create_product(db, pim_id, fields)
+        product, was_created = await lookup_or_create_product(db, pim_id, fields, tenant_id=tenant_id)
         row["product_id"] = str(product.id)
         row["product_created"] = was_created
         # Carry forward any STEP file already linked to this product