Accept a start_kernel request and return an operation_id immediately.

The get_user_filesystem dependency chains: OAuth session/internal JWT → user → filesystem. The user's filesystem path is injected into the body before forwarding to the upstream gateway, which bind-mounts it into the Singularity container.

If the kernel is already running, emits kernel_alive and returns immediately — mirroring the local backend's behaviour and avoiding a misleading "Kernel starting…" / "Kernel started ✓" toast cycle.

Poll GET /operations/{operation_id} for the result.