Add CI test-coverage gate at 80%, backfill remaining logic-module gaps

pytest-cov + pyproject.toml [tool.coverage.*] measure reporting.py,
nightscout_client.py, data_processing.py, pdf_export.py, profile_data.py,
ui_common.py, and auth.py; app.py and pages/*.py are omitted (thin
Streamlit UI scripts, not meaningfully unit-testable without a running
Streamlit session — the handful of similarly UI-coupled functions left
in ui_common.py/auth.py are marked "# pragma: no cover" with the same
rationale, since the logic they call into is already fully tested).

The Gitea Actions test job now runs with --cov and fails the build
below 80%; the coverage XML is uploaded as a build artifact and fed
into the SonarQube scan (closing the gap flagged in
homelab-cicd-werkinstructie.md). Backfilled a few missing branches
(get_colors() theme fallback, secondary-y-axis overlay traces) to reach
98% on the measured modules — comfortable headroom above the 80% gate.

Builds on #29/#30/#31.

README.md

@@ -41,3 +41,12 @@ klinische rapportagepagina's (zie `docs/requirements-reporting.md` en
 pytest tests/ -v
 ruff check .
 ```
+
+Coverage (zie `pyproject.toml` se `[tool.coverage.*]`): de CI-pipeline faalt
+als de dekking op de gemeten modules onder 80% zakt (`app.py`/`pages/*.py`
+zijn uitgesloten — dunne Streamlit-UI-scripts, niet zinvol te unit-testen
+zonder een draaiende Streamlit-sessie).
+
+```bash
+pytest tests/ --cov=. --cov-report=term-missing
+```