fix: robust colored-text detection in graphic filter

The 25x25 dilation kernel merges nearby green words into large regions,
so pixel-overlap with OCR word boxes drops below 50%. Previous density
checks alone weren't sufficient.

New multi-layered approach:
- Count OCR word CENTROIDS inside each colored region
- ≥2 centroids → definitely text (images don't produce multiple words)
- 1 centroid + 10%+ pixel overlap → likely text
- Lower pixel overlap threshold from 50% to 40%
- Raise density+height thresholds for text-line detection
- Use INFO logging to diagnose remaining false positives

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

klausur-service/backend/cv_graphic_detect.py

@@ -181,40 +181,71 @@ def detect_graphic_elements(
         word_pixel_count = int(np.sum(roi_words > 0))
         word_overlap = word_pixel_count / bbox_area if bbox_area > 0 else 0
 
+        # Check: how many OCR word centroids fall inside this region?
+        # Colored text that OCR detected will have multiple centroids inside.
+        # Actual images may have 0-1 spurious OCR artifacts.
+        word_centroid_count = sum(
+            1 for wb in word_boxes
+            if (bx <= int(wb.get("left", 0) + wb.get("width", 0) / 2) <= bx + bw
+                and by <= int(wb.get("top", 0) + wb.get("height", 0) / 2) <= by + bh)
+        )
+
         # Check: how many actual colored pixels are in this region?
         roi_color = color_pixel_raw[by:by + bh, bx:bx + bw]
         color_pixel_count = int(np.sum(roi_color > 0))
 
-        # If most of the region is covered by word boxes → colored text, skip
-        if word_overlap > 0.5:
-            logger.debug("GraphicDetect PASS1 skip text region (%d,%d) %dx%d overlap=%.0f%%",
-                         bx, by, bw, bh, word_overlap * 100)
+        # Color pixel density (before any skip checks so we can log it)
+        density = color_pixel_count / bbox_area if bbox_area > 0 else 0
+
+        # --- Skip heuristics for colored TEXT (not images) ---
+
+        # (a) High word-box pixel overlap → clearly text
+        if word_overlap > 0.40:
+            logger.info(
+                "GraphicDetect PASS1 skip text-overlap (%d,%d) %dx%d "
+                "overlap=%.0f%% centroids=%d",
+                bx, by, bw, bh, word_overlap * 100, word_centroid_count,
+            )
+            continue
+
+        # (b) Multiple OCR words detected inside → colored text
+        #     (images rarely produce 2+ confident word detections)
+        if word_centroid_count >= 2:
+            logger.info(
+                "GraphicDetect PASS1 skip multi-word (%d,%d) %dx%d "
+                "centroids=%d overlap=%.0f%% density=%.0f%%",
+                bx, by, bw, bh, word_centroid_count,
+                word_overlap * 100, density * 100,
+            )
+            continue
+
+        # (c) Even 1 word + some pixel overlap → likely text
+        if word_centroid_count >= 1 and word_overlap > 0.10:
+            logger.info(
+                "GraphicDetect PASS1 skip word+overlap (%d,%d) %dx%d "
+                "centroids=%d overlap=%.0f%%",
+                bx, by, bw, bh, word_centroid_count, word_overlap * 100,
+            )
             continue
 
         # Need a minimum number of colored pixels (not just dilated area)
         if color_pixel_count < 200:
             continue
 
-        # Color pixel density: fraction of bbox filled with colored pixels.
-        # Text strokes are thin → low density (typically 5-20%).
-        # Actual images/graphics are filled → high density (30%+).
-        density = color_pixel_count / bbox_area if bbox_area > 0 else 0
-
-        # Very low density → almost certainly colored text, not an image
+        # (d) Very low density → thin strokes, almost certainly text
         if density < 0.20:
-            logger.debug(
+            logger.info(
                 "GraphicDetect PASS1 skip low-density (%d,%d) %dx%d "
                 "density=%.0f%% (likely colored text)",
                 bx, by, bw, bh, density * 100,
             )
             continue
 
-        # Moderate density + small height → likely a colored text line
-        # (text-line height is typically < 3% of page height)
-        if density < 0.30 and bh < h * 0.04:
-            logger.debug(
-                "GraphicDetect PASS1 skip text-height region (%d,%d) %dx%d "
-                "density=%.0f%% height=%.1f%% (likely colored text line)",
+        # (e) Moderate density + small height → colored text line
+        if density < 0.35 and bh < h * 0.05:
+            logger.info(
+                "GraphicDetect PASS1 skip text-height (%d,%d) %dx%d "
+                "density=%.0f%% height=%.1f%%",
                 bx, by, bw, bh, density * 100, 100.0 * bh / h,
             )
             continue