fix: skip recordDigestSent when RESEND_API_KEY absent (closes #2415)

[Rudrasamadhiya]

When RESEND_API_KEY is absent, sendEmail previously returned { ok: true } which caused recordDigestSent to be called, advancing last_digest_sent_at even though no email was actually sent. On the next cron run, these users would be silently skipped until the next weekly window.

This PR adds a skipped flag to the sendEmail return type so the cron handler can detect when the email step was a no-op and skip the timestamp update accordingly.

Changes

• Added skipped?: boolean to sendEmail return type
• • Return { ok: true, skipped: true } when API key is missing
• • Guard recordDigestSent call with !result.skipped
    Closes [BUG] Weekly digest cron treats missing RESEND_API_KEY as a successful send, consuming the 6-day cooldown with no email sent #2415

[github-actions]

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