fix(mailer): use logger.log instead of debug for transporter ready

[shishir-subedii]

Summary

Currently, when the mail transporter is successfully verified, the message
Transporter is ready is logged with logger.debug.

This means developers only see this confirmation when debug-level logging
is enabled. However, the transporter being ready is an important success
state that should be visible under standard logging.

Change

• Replaced this.mailerLogger.debug with this.mailerLogger.log for the
  "Transporter is ready" message.

Reasoning

• Improves visibility: Developers always know when the transporter is ready.
• Consistency: Startup success messages in NestJS typically use log().
• No breaking change: Only affects log visibility, not functionality.

Notes

If maintainers prefer not to show this message at log level, an alternative
would be to make the log level configurable.