fix: prevent digest cooldown consumption when RESEND_API_KEY is missing

[Sricharan106]

Summary

Fixes an issue where the weekly digest cron reported emails as successfully sent when RESEND_API_KEY was not configured. Previously, users were marked as having received a digest and entered the 6-day cooldown period despite no email being delivered.

Closes #2415

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

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What Changed

• Updated sendEmail() to distinguish between successfully sent and skipped emails when RESEND_API_KEY is not configured.
• Prevented last_digest_sent_at from being updated for skipped emails.
• Added separate tracking for skipped emails so they do not consume the weekly digest cooldown period.
• Corrected cron response metrics to avoid reporting emails as sent when no delivery occurred.

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How to Test

1. Run the weekly digest cron with RESEND_API_KEY unset.
2. Verify the cron completes without sending emails.
3. Confirm users' last_digest_sent_at values are not updated and that skipped emails are reported separately.

Expected result:

• No emails are sent.
• Users are not placed into the cooldown window.
• sentCount remains accurate.
• Skipped deliveries are tracked separately.

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Additional Context

Before this change, deployments without a configured Resend API key could report successful email delivery while silently preventing users from receiving future digests due to the cooldown mechanism. This fix ensures cooldown timestamps are only recorded after a real email delivery attempt succeeds.

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[Sricharan106]

@Priyanshu-byte-coder can you review the pr when you got time