chore: make coverage-check a realistic gate and add coverage-gaps helper

[mrofreP]

The coverage-check target enforced a 95% threshold the codebase has
never met -- measured per-package coverage is 74.1% -- so the gate
always failed and was effectively ignored.

Lower the threshold to a COVERAGE_MIN floor of 70 (overridable), just
below the current 74.1%, documented as a floor to raise rather than
lower. On failure the message now points at coverage-report and the new
coverage-gaps helper instead of only exiting.

coverage-gaps lists the 20 least-covered functions so contributors can
see where to invest in tests. Coverage is measured per-package -- the
same coverage.out that make test produces -- so packages with no unit
tests (the generated mocks and OpenAPI stubs) are absent from the
profile rather than dragging the number down; no separate exclusion is
needed.

Also gitignore the coverage.txt and coverage.html artifacts the coverage
targets produce.

[codacy-production]

Up to standards ✅

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