Seed signup history from existing users, streaming progress (SSE). Admin only.

The table ships empty, which would let every pre-existing user delete their account and re-register for a second signup grant. This walks ta_user (ordered by creation date) and inserts one history row per email, marked as already granted.

Idempotent and resumable: rows that exist are left untouched, so re-running is a no-op. Keep calling with the returned next_offset until scanned comes back as 0.