ci: fix backend bump automation

[MilesCranmerBot]

This updates the backend bump automation to:

• update .release-please-manifest.json alongside pyproject.toml and pysr/juliapkg.json
• use conventional-commit-style PR titles and commit messages
• automatically dispatch the usual CI workflows for backend-update PRs created by the workflow

This also adds workflow_dispatch to the workflows that need to be triggered manually from the backend-update automation.

Context: this was requested from #1157 (comment).

[MilesCranmer]

@MilesCranmerBot it says "This branch has conflicts that must be resolved
" - please fix. Also the pre-commit ci is broken

[MilesCranmerBot]

Fixed.

• merged master into this PR branch to clear the conflict
• resolved the workflow conflict in .github/workflows/update_backend.yml
• kept the expanded backend-update CI triggers
• ran pre-commit on the touched workflow files (check-yaml, black, pycln, isort, etc.)

Commit: ef1ff5d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.25%. Comparing base (cdee919) to head (ef1ff5d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1166   +/-   ##
=======================================
  Coverage   94.25%   94.25%           
=======================================
  Files          21       21           
  Lines        1687     1687           
=======================================
  Hits         1590     1590           
  Misses         97       97           

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