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	7ac5912
🔍 Latest deploy log	https://app.netlify.com/projects/endearing-brigadeiros-63f9d0/deploys/699c49584937780008a24a58

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.78%. Comparing base (692531b) to head (7ac5912).
⚠️ Report is 13 commits behind head on main.

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   81.39%   81.78%   +0.39%     
==========================================
  Files          67       67              
  Lines        4741     4766      +25     
  Branches      819      827       +8     
==========================================
+ Hits         3859     3898      +39     
+ Misses        867      852      -15     
- Partials       15       16       +1     

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[jescalada]

Tested the core flow and edge cases, LGTM!

Just a heads up, a fair bit of the attestation endpoint logic has been improved in #1323. Attestation-related classes have been added as well. I'm able to handle conflicts and extra changes to the /reject endpoint in that PR - it'd be great if you could take a look and make sure things still work as expected!

[andypols]

@jescalada happy to check out #1323

[jescalada]

@andypols Thank you! I'll ping you after fixing up the conflicts, likely tomorrow...