ci: Add mypy problem matcher

[khvn26]

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Changes

This wraps the make typecheck call in CI with a problem matching action, enabling Mypy annotations for the PRs.

How did you test this code?

Tested in CI with a deliberate typing error:

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Jul 2, 2025 4:16pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Jul 2, 2025 4:16pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Jul 2, 2025 4:16pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-5692	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-5692	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-5692	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-5692	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5692	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5692	Finished ✅	Results ✅

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.77%. Comparing base (de7c278) to head (f2f4c85).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5692   +/-   ##
=======================================
  Coverage   97.77%   97.77%           
=======================================
  Files        1253     1253           
  Lines       44242    44242           
=======================================
  Hits        43256    43256           
  Misses        986      986           

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