[INTS26] Set up Mailer/Send Bulk Rejection Emails

[OwenChen07]

Notion ticket link

Set up Mailer/Send Bulk Rejection Emails

Implementation description

• Kept the boilerplate Nodemailer + Gmail OAuth2 transport (emailService.ts) rather than switching providers, and extended it with sendBulkEmail, which sends in batches of 10 and never rejects for individual failures — it returns { sent, failed } so one bad address can't abort a whole rejection run.
• Added a file-based HTML template system (emails/templates/*.html + emails/index.ts) using Handlebars. Each email type is one HTML file with {{placeholders}} plus one typed builder function (e.g. buildRejectionEmail), so future devs can add new templates without touching the sending logic.
• Wired bulk rejection emails into the existing bulkUpdateApplicantRecordsStatus mutation: after the status update transaction commits, any records set to REJECTED get a templated rejection email fetched with their Applicant data. This is fire-and-forget — email failures are logged but never roll back the already-committed status change, and the mutation doesn't wait on send completion.
• Documented Gmail OAuth2 credential setup (including the invalid_grant/Testing-mode-7-day-expiry gotcha) and "how to add a new email template" in docs/mailer.md.

Steps to test

1. Run the new unit tests: cd backend/typescript && npx jest emails emailService.test applicantRecordService.test (6 tests covering template rendering, bulk-send partial failure handling, and the rejection hook's message-building/error-resilience).
2. Preview the template with sample data using this command:
   """
   npx ts-node -e 'import { buildRejectionEmail } from "./emails";
   const { html } = buildRejectionEmail({ firstName: "Owen", position: "Developer", term: "Fall 2026" });
   require("fs").writeFileSync("/tmp/rejection-preview.html", html);'
   open /tmp/rejection-preview.html
   """
3. Real send test using this command*:
   *The credentials in .env have expired, so I can send you the new credentials I created on Discord if you want.
   """
   docker exec -i recruitment_platform_ts_backend npx ts-node -e '
   import EmailService from "./services/implementations/emailService";
   import nodemailerConfig from "./nodemailer.config";
   import { buildRejectionEmail } from "./emails";
   const svc = new EmailService(nodemailerConfig, "UW Blueprint Recruitment");
   const { subject, html } = buildRejectionEmail({ firstName: "Owen", position: "Developer", term: "Fall 2026" });
   svc.sendEmail("owenchenyp@gmail.com", subject, html).then(() => console.log("sent!")).catch(console.error);'
   """

What should reviewers focus on?

• The credentials in .env have expired, so someone will need to create new ones. There are instructions in recruitment-platform/docs/mailer.md.

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR