feat: Add reject reason

[andypols]

Closes #605.

Summary

Right now, reviewers can reject a push without leaving any context, and push authors can’t tell who rejected it or why. This PR makes rejections explicit by requiring a reason and recording the rejecting reviewer, then surfaces that information in the UI.

UI

• Adds a rejection modal (modeled after the existing Attestation modal) that requires a reviewer to enter a reason before a rejection can be submitted.

• Displays rejection details on rejected pushes, including who rejected and why, using an info box pattern consistent with the Attestation UI.

• Refactors PushDetails by extracting components to reduce complexity and improve readability/maintainability.

Backend

• Introduces a Rejection type containing reviewer, reason, and timestamp.
• Persists rejections in the DB layer for both file-based storage and MongoDB.
• Adds server-side validation to enforce that a rejection reason is provided.
• Adds unit tests covering the rejection persistence flow.
• Updates integration tests to include a rejection reason.

[netlify]

✅ Deploy Preview for endearing-brigadeiros-63f9d0 canceled.

Name	Link
🔨 Latest commit	e376663
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/6989e0c9d8b4120008b8b1de

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.74%. Comparing base (784bfc4) to head (e376663).

Files with missing lines	Patch %	Lines
src/service/routes/push.ts	74.07%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1396      +/-   ##
==========================================
+ Coverage   81.34%   81.74%   +0.39%     
==========================================
  Files          65       65              
  Lines        4648     4673      +25     
  Branches      792      800       +8     
==========================================
+ Hits         3781     3820      +39     
+ Misses        852      837      -15     
- Partials       15       16       +1     

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