Billing→S3: BillingS3LogHandler (buffer, flusher, metrics)

[Hazel-Datastax]

The custom log handler — receives billing.events records, batches them off the request path, and flushes .jsonl files to S3 using its own overflow policy.

What to do

Add BillingS3LogHandler extends java.util.logging.Handler:

• publish(LogRecord): extract the message, enqueue into an own bounded buffer; non-blocking, never throws; on full → drop + Micrometer counter.
• Background flusher: seal per-event_type batches (Billing→S3: batch + S3 key builder from logged billing lines #2516) on threshold → serialize → upload (Billing→S3: S3 uploader + LocalStack IT #2517) with idempotent key.
• flush() / close(): drain remaining batches (used on graceful shutdown).
• Micrometer metrics: offered / flushed / dropped / failed counters + queue-depth gauge.

Acceptance

• Unit tests: thresholds seal a batch; full buffer drops + increments the counter; close() drains the remainder; uploader invoked with the expected key + content.