ci: fix Codecov uploads (token + scope coverage to the app package)

[fabriziosalmi]

The README coverage badge was stuck at a stale ~38% that matched no real measurement. Root cause: Codecov rejected every upload from the protected main branch —

-> Token length: 0
error -- Upload queued for processing failed: {"message":"Token required because branch is protected"}

The codecov-action ran without a token, and with no fail_ci_if_error the upload step failed silently while the job stayed green, so the badge froze at the last successful upload (months old).

Real coverage today: ~70% over modules/ (84% if you count the tests/ tree, which --cov=. was folding in). So this is a broken-badge problem, not a low-coverage problem.

Changes

• token: ${{ secrets.CODECOV_TOKEN }} on the codecov-action so uploads from main succeed again. (CODECOV_TOKEN repo secret added separately.) fail_ci_if_error stays false so a transient Codecov outage never blocks a release.
• --cov=modules instead of --cov=. — measure the application package, not the whole repo (the tests/ tree runs at ~100% and was inflating the number). The badge will report the honest ~70%.
• codecov.yml (new): informational statuses (no PR gating on a single-maintainer project) + an ignore list for tests/scripts/docs.

After this merges, the next CI run on main uploads successfully and the badge updates to the real number.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.39%. Comparing base (4eacfad) to head (5ce0353).
⚠️ Report is 263 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #317       +/-   ##
===========================================
+ Coverage   38.41%   70.39%   +31.97%     
===========================================
  Files          69       49       -20     
  Lines        8981    11244     +2263     
===========================================
+ Hits         3450     7915     +4465     
+ Misses       5531     3329     -2202     

Flag	Coverage Δ
unittests	70.39% <ø> (+31.97%)	⬆️

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