Complete long-context chat support

migrations/20260724233000_add_session_context/down.sql

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+ALTER TABLE sessions DROP COLUMN context_limit;
+ALTER TABLE sessions DROP COLUMN context_used;

migrations/20260724233000_add_session_context/up.sql

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+ALTER TABLE sessions ADD COLUMN context_used INTEGER NOT NULL DEFAULT 0 CHECK (context_used >= 0);
+ALTER TABLE sessions ADD COLUMN context_limit INTEGER NOT NULL DEFAULT 0 CHECK (context_limit >= 0);