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/render_dispatcher.py

@@ -1,3 +1,9 @@
-# Compat shim — use app.domains.rendering.service instead
-from app.domains.rendering.service import dispatch_render
+# Compat shim — routes to render_order_line_task (the working implementation)
+def dispatch_render(order_line_id: str) -> dict:
+    """Queue render_order_line_task for the given order line."""
+    from app.tasks.step_tasks import render_order_line_task
+    render_order_line_task.delay(order_line_id)
+    return {"backend": "celery", "queued": True}
+
+
 __all__ = ["dispatch_render"]