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/domains/rendering/service.py

@@ -1,6 +1,7 @@
 """Rendering services — template resolution, dispatch, and Blender utilities."""
 
-# Re-export from original service files for backward compatibility.
+# Re-export from canonical service files for backward compatibility.
+# template_service contains the actual sync implementations (Celery-safe).
 from app.services.template_service import resolve_template, get_material_library_path
 from app.services.render_dispatcher import dispatch_render
 from app.services.render_blender import find_blender, is_blender_available