Logs hardcoded timestamps as Unix milliseconds, which carries no
timezone and reads as UTC in plain log viewers (podman logs, logread).
This adds a logTimeFormat config field so operators can pick a
TZ-aware format, while keeping the existing numeric output as default.

- New TypeLogTimeFormat wrapper (typed like every other config field):
  presets unix / unix-ms / unix-micro / unix-nano / rfc3339 /
  rfc3339-nano, and any other value is passed through to zerolog as a
  Go reference-time layout. Set() rejects an empty value.
- Default stays "unix-ms" -> zerolog.TimeFormatUnixMs, so output is
  byte-for-byte unchanged unless the option is set.
- run_proxy.go resolves the configured value instead of hardcoding it.
- example.config.toml documents the presets with a single consistent
  example instant.

Based on the approach of #531 by @resolvicomai. Resolves #427.