Add assignment email digests

[Jorel97]

Fixes #6.

Summary

• add optional assignee email capture and persist normalized assignee emails with tasks
• queue assignment changes into hourly digest batches instead of sending one email per assignment
• add stable unsubscribe links that clear pending digests and block future assignment notifications
• support a local log transport by default plus Resend delivery via environment variables
• document provider setup and cover digest/unsubscribe behavior with tests

Verification

• npm test
• npm run lint
• node --check server.js
• node --check src/app.js
• node --check src/tasks.js
• node --check test/server.test.js
• git diff --check

[codebounty-dev-bot]

⚠️ Attention Sponsor: This PR was submitted by @Jorel97 who has not applied for this bounty (#6). Please verify if you want to consider this contribution.

👋 Hey @Jorel97: This issue has a bounty on CodeBounty! To be eligible for the reward, please sign up on CodeBounty and apply for this bounty. If you apply, your PR will be automatically associated with your application.

[Jorel97]

Hi CodeBounty/sponsor team, I logged in to CodeBounty as @Jorel97 and opened the bounty page for #6, but it currently shows Pending_approval, Applications (0), and no Apply button is available.

This PR was submitted for the $200 bounty on #6. Could you please confirm whether the bounty needs maintainer approval before applications can be submitted, and whether this PR can be associated with my application once the bounty is open?