feat: migrate from Forgejo to GitLab CI with test reporting

- Convert Forgejo workflow to GitLab CI/CD (.gitlab-ci.yml) - Add JUnit XML and coverage reporting
with pytest-cov - Configure caching and artifacts for better CI performance - Fix pytest.ini
configuration and enhance .coveragerc - Support GitLab v18.1.1 with comprehensive test visualization

.coveragerc

@@ -1,8 +1,12 @@
 [run]
 source = doi2dataset
-omit = 
+omit =
     */tests/*
+    */test_*
     */docs/*
+    */__pycache__/*
+    */venv/*
+    */.venv/*
     setup.py
     conf.py
     __init__.py
@@ -11,13 +15,22 @@ omit =
 exclude_lines =
     pragma: no cover
     def __repr__
+    def __str__
     if self.debug:
     raise NotImplementedError
+    raise AssertionError
     if __name__ == .__main__.:
+    if TYPE_CHECKING:
+    @abstractmethod
     pass
     raise ImportError
     except ImportError
-    def __str__
+
+show_missing = true
+precision = 2
 
 [html]
-directory = htmlcov
+directory = htmlcov
+
+[xml]
+output = coverage.xml