[ING-480] misc(event): Add miliseconds precision to events_raw ingested_at (#6044)

## Context

`ingested_at` is stamped when the API receives an event and is stored in
ClickHouse as a millisecond-precision timestamp. It was serialized
without fractional seconds, so the value was truncated to the whole
second before being written.

This makes the ingestion pipeline latency impossible to measure. The
propagation delay is often sub-second, so the truncation hides the real
distribution and biases every reading upwards by up to one second. It
also produces negative delays, because
a truncated ingestion time can precede the moment the enriched event was
written.

That measurement is needed to size the intervals driving wallet and
usage refreshes: without it, propagation cannot be separated from
scheduling.

## Description

Serialize `ingested_at` with millisecond precision on both produce
paths, regular ingestion and re-enrichment.

The column type is unchanged, so only the written value becomes more
precise. Existing rows keep their truncated values, and consumers
already accept a fractional part, so nothing downstream needs to change.