Raise color sat_threshold from 50 to 55 to avoid scanner blue artifacts

Black text has median_sat ~6-7, green text ~63-65. At threshold 50,
scanner blue tints (median_sat ~50-54) on words like "Wasser" were
falsely classified as blue. Threshold 55 has good margin on both sides.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/backend/cv_color_detect.py

@@ -81,7 +81,7 @@ def _hue_to_color_name(hue: float) -> str:
 def detect_word_colors(
     img_bgr: np.ndarray,
     word_boxes: List[Dict],
-    sat_threshold: int = 50,
+    sat_threshold: int = 55,
     min_sat_ratio: float = 0.25,
 ) -> None:
     """Annotate each word_box in-place with its detected text color.