feat: record token counts and the producing model per message

Cost has to survive reopening a conversation, which means storing what
each reply used. The model goes on the message rather than the session
because sessions records only the current one, and a conversation that
switched models would otherwise be priced entirely at whichever is
selected now. All three columns are nullable so old rows read as unknown.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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