fix(measurements): export CSV ammette admin senza ruolo esplicito (#36 punto 3)

Il controllo ruolo di GET /api/measurements/export/csv ignorava is_admin:
un admin senza MeasurementTec/Metrologist assegnato prendeva 403, a
differenza di ogni altro endpoint che passa da require_role (ruolo o admin).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/api/routers/measurements.py

@@ -274,8 +274,11 @@ async def export_measurements_csv(
     db: AsyncSession = Depends(get_db),
 ):
     """Export measurements to CSV with configurable delimiter and decimal separator."""
-    # Check user has MeasurementTec or Metrologist role
-    if not (user.has_role("MeasurementTec") or user.has_role("Metrologist")):
+    # Check user has MeasurementTec or Metrologist role, same as require_role: an
+    # admin with no explicit role assigned is still let through (#36 point 3).
+    if not (
+        user.has_role("MeasurementTec") or user.has_role("Metrologist") or user.is_admin
+    ):
         raise HTTPException(
             status_code=status.HTTP_403_FORBIDDEN,
             detail="MeasurementTec or Metrologist role required",