`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.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.