fix: store per-turn token sums so searched turns price every round

A searched turn makes one API call per search round and all of them are
billed, but prompt_tokens/completion_tokens held only the last round's
counts, so a searched turn under-reported its cost. The columns now hold
the sum across rounds; usage_json keeps each round's raw usage block as
a JSON array for external consumers to verify against. A single-round
turn stores the same numbers as before.

0 files changed, 0 insertions, 0 deletions