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/api/routers/products.py

@@ -206,7 +206,7 @@ async def create_product(
         raise HTTPException(409, detail=f"Product with pim_id '{body.pim_id}' already exists")
 
     from app.services.product_service import create_default_positions
-    product = Product(**body.model_dump())
+    product = Product(**body.model_dump(), tenant_id=getattr(user, 'tenant_id', None))
     db.add(product)
     await db.flush()
     await create_default_positions(db, product.id)