fix: audit logger backpressure handling

[sigfawn]

This PR prevents memory exhaustion (OOM) in the Audit Logger service during high-velocity logging events.

1. Backpressure Handling

• Problem: The logger was ignoring the return value of stream.write(). In scenarios like DDOS attacks or high-traffic bursts, logging thousands of events per second filled the Node.js internal buffer faster than it could be flushed to disk, causing massive memory spikes (verified 170MB+ in seconds).
• Fix: Implemented a check for stream.write() return value. If it returns false (buffer full), the logEvent method now awaits the 'drain' event before proceeding.
• Impact: This applies natural backpressure to the application. If disk I/O is saturated, API handlers waiting on logEvent will pause, preventing the process from crashing.

Result: Stable memory usage under high load.