feat: add code coverage with branch coverage

- Add --enable-coverage configure option
- Add lcov targets for generating coverage reports
- Enable branch coverage (--rc branch_coverage=1)
- Install lcov in all Docker images
- Coverage report displayed via lcov --summary (no Codecov - doesn't work with Gitea)

.github/workflows/ci-code.yml

@@ -121,10 +121,5 @@ jobs:
               --output-file /coverage/coverage.info \
               --rc branch_coverage=1 \
               --ignore-errors inconsistent,empty,unused
-          '
-      - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v4
-        with:
-          files: ./coverage/coverage.info
-          fail_ci_if_error: false
-          verbose: true
+            lcov --summary /coverage/coverage.info
+          '