Show estimated HbA1c in mmol/mol (IFCC) alongside the % value

Dutch lab results conventionally report HbA1c in mmol/mol rather than
DCCT/NGSP %. Adds gmi_mmol_per_mol() using the standard IFCC conversion
(Hoelzel et al. 2004) and shows it next to the existing % value on the
Samenvatting page and in the PDF summary.

Closes gitea #25.

reporting.py

@@ -36,6 +36,17 @@ def gmi_percent(mean_mmol: float) -> float:
     return round(3.31 + 0.4311 * mean_mmol, 1)
 
 
+def gmi_mmol_per_mol(gmi_pct: float) -> int:
+    """Zet een GMI-percentage (DCCT/NGSP) om naar IFCC mmol/mol.
+
+    mmol/mol = 10.929 x (% - 2.15) — de standaard IFCC-omrekening
+    (Hoelzel et al. 2004, Clin Chem 50(1):166-174), gangbare eenheid in
+    Nederlandse labuitslagen naast het %-getal (gitea #25). Bijv. 6.5% =
+    48 mmol/mol, de bekende diagnostische drempel.
+    """
+    return round(10.929 * (gmi_pct - 2.15))
+
+
 GMI_TREND_COLUMNS = ["date", "gmi", "rolling_avg", "trend", "trend_lower", "trend_upper"]