feat: finalized support for chat

migrations/20260724234000_add_session_generation_speed/down.sql

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+ALTER TABLE sessions DROP COLUMN last_tokens_per_second;

migrations/20260724234000_add_session_generation_speed/up.sql

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+ALTER TABLE sessions ADD COLUMN last_tokens_per_second FLOAT CHECK (last_tokens_per_second IS NULL OR last_tokens_per_second >= 0);