fix: harden mailer transport selection and fallbacks

[kaizercodes]

Overview

This PR hardens the transport selection and fallback logic in src/lib/mailer.ts so that disabled, console, SMTP, and missing-nodemailer paths behave predictably and observably. The mailer is on the critical path for quota and invoice notifications, so silent or ambiguous fallback behavior is a real risk; this change makes each branch explicit and logs the outcome through the shared logger.

Related Issue

Changes

📬 Mailer transport selection and fallbacks

• [MODIFY] src/lib/mailer.ts
  • Clarified the sendMail control flow so each transport mode is handled in a single, explicit branch:
    • Disabled mode returns early without attempting any transport and emits a skip log via src/logger.ts.
    • Console transport logs the message payload, omitting the body in production so sensitive content is not written to production logs.
    • SMTP transport dynamically imports nodemailer, constructs the transporter, and calls transporter.sendMail with from, to, and subject (plus body) populated from the message.
    • Missing nodemailer is caught at the dynamic import boundary and falls back to the console transport, emitting a warning so the degraded state is visible in logs rather than silent.
  • Kept the existing configureMailer entry point as the single place to toggle modes, so behavior is driven by configuration rather than ad-hoc branching.
  • Preserved existing safeguards: no transport is invoked when disabled, and the fallback path never throws on a missing optional dependency.

Verification Results

npm test -- src/lib/mailer.test.ts

Manual review of each branch in src/lib/mailer.ts confirms:

• Disabled mode returns before any transport call and logs a skip.
• Console transport logs the payload with the body redacted in production.
• SMTP mode calls transporter.sendMail with from/to/subject.
• Missing nodemailer falls back to console and logs a warning.

Acceptance Criteria	Status
Disabled mode sends nothing and logs a skip	✅ Early return before transport; skip emitted via logger
Console transport logs the payload without the body in production mode	✅ Body omitted from production console output
SMTP mode calls transporter.sendMail with from/to/subject	✅ Dynamic import + sendMail({ from, to, subject, ... })
Missing nodemailer falls back to console and logs a warning	✅ Import failure caught; console fallback + warning log

Security and Failure-Mode Handling

• Production log hygiene: the console transport intentionally drops the message body in production to avoid leaking notification content (quota/invoice details) into log aggregation.
• No silent degradation: the missing-nodemailer path emits a warning, so operators can detect that SMTP delivery has degraded to console instead of discovering dropped customer emails after the fact.
• No weakened validation: disabled mode still short-circuits before any transport is touched, and the fallback path only triggers on import failure — it does not mask SMTP send errors.

Compatibility

• No public API changes: configureMailer and sendMail keep their existing signatures.
• nodemailer remains an optional dependency resolved via dynamic import; environments without it continue to work through the console fallback.
• No dependency upgrades or unrelated refactors.

Closes #1295

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