Previously, _initialize_batch forced use_ephemeral=True for all
first-time users, bypassing _check_authenticated() entirely. This
meant logged-in users in a new project directory were routed to the
ephemeral endpoint instead of their account's tracing endpoint.
Now _check_authenticated() runs for all users including first-time.
Authenticated first-time users get non-ephemeral tracing (traces
linked to their account); only unauthenticated first-time users
fall back to ephemeral. The deferred backend init in
FirstTimeTraceHandler also reads is_current_batch_ephemeral instead
of hardcoding use_ephemeral=True.
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