ci: Allow manually dispatching the master Checks workflow

[vdusek]

Summary

Adds workflow_dispatch to CI (master) so the Checks suite can be re-run manually on the same commit when the push-triggered run is lost — e.g. during a GitHub outage. The doc_release / beta_release jobs are gated on github.event_name == 'push' so manual dispatches only re-run the checks and never re-publish.

The redundant workflow_dispatch trigger on _checks.yaml is removed: dispatching it directly produced check-run names without the Checks / prefix, so the wait-for-checks gate in the manual release workflows wouldn't recognize them anyway.

How to use

When a push-triggered Checks run on master is missing, go to Actions → CI (master) → Run workflow on master. Once it succeeds, the manual release workflows' wait-for-checks gate will be satisfied.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.92%. Comparing base (12ef1cb) to head (b35c30e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1917   +/-   ##
=======================================
  Coverage   92.91%   92.92%           
=======================================
  Files         167      167           
  Lines       11714    11714           
=======================================
+ Hits        10884    10885    +1     
+ Misses        830      829    -1     

Flag	Coverage Δ
unit	92.92% <ø> (+<0.01%)	⬆️

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