fix(backend): log mail delivery failures

[dollarop]

Summary

• log a warning when SMTP configuration is missing
• log nodemailer import failures and delivery failures with recipient and mail type context
• avoid logging SMTP credentials or message content
• add regression tests for previously silent failure paths

Closes #454

Verification

• npm test -- --runInBand src/services/__tests__/mailerService.test.ts (2 tests passed)
• npm run build remains blocked by pre-existing syntax errors in backend/src/services/tenantConfigService.ts:207 and nearby lines