feat: customize log level

[filipgolonka]

@wiggin77 🙏

[Copilot]

Pull Request Overview

This PR adds configurable log level support to the mail relay application. Instead of hardcoding the log level to "info", users can now specify their desired log level in the configuration file.

• Added log_level field to the configuration structure and JSON schema
• Modified logger setup to use the configured log level instead of hardcoded "info"
• Set "info" as the default log level in configuration defaults

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
testdata/valid.json	Added example log_level configuration field
main.go	Added LogLevel field to config struct, updated setupLogger function signature, and set default value
server.go	Updated to use configured log level instead of hardcoded value

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[wiggin77]

LGTM 👍 Thanks @filipgolonka