ci: improve workflow robustness and coverage

[martin-belanger]

• meson-test: add ubuntu-22.04 to the test matrix, weekly scheduled run, and a 60-minute timeout
• linters: add per-job timeouts (10/30 min)
• dependabot: track pip dependencies
• README: add CodeQL status badge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.72%. Comparing base (b934cff) to head (8ec7e4f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #476   +/-   ##
=======================================
  Coverage   58.72%   58.72%           
=======================================
  Files          16       16           
  Lines        2612     2612           
=======================================
  Hits         1534     1534           
  Misses       1078     1078           

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