Add backpressure to CommandExecutor write queue

[jessica-claude]

Description

CommandExecutor._write_queue is an unbounded asyncio.Queue(). If the database is slow or the service has failed, execution records accumulate without bound until OOM.

Acceptance Criteria

• _write_queue has a bounded maxsize (e.g., 10000)
• When full, records are dropped with a rate-limited warning log
• Queue depth is observable (periodic log or metric)