`kafka_event_aggregator`: backpressure

If producer queue fills beyond N bytes, pause consumer until it recovers to less than M bytes (and provide warnings + metrics about consumer pause).

(need to consider behaviour on pausing consumer mid-frame - we are likely to get partial frames in that case)

Start at the kafka_event_aggregator entry point and trace how producer queue size, consumer reads, and frame boundaries are handled. Define the N/M thresholds and confirm how pausing mid-frame should work; done means consumer pause and recovery, warnings, and pause metrics are implemented without losing or mishandling partial frames.

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