fix: hard-filter OCR words inside detected graphic regions

Run detect_graphic_elements() in the grid pipeline after image loading
and remove ALL words whose centroids fall inside detected graphic regions,
regardless of confidence. Previously only low-confidence words (conf < 50)
were removed, letting artifacts like "Tr", "Su" survive.

Changes:
- grid_editor_api.py: Import and call detect_graphic_elements() at Step 3a,
  passing only significant words (len >= 3) to avoid short artifacts fooling
  the text-vs-graphic heuristic. Hard-filter all words in graphic regions.
- cv_graphic_detect.py: Lower density threshold from 20% to 5% for large
  regions (>100x80px) — photos/illustrations have low color saturation.
  Raise page-spanning limit from 50% to 60% width/height.

Tested: 5 ground-truth sessions pass regression (079cd0d9, d8533a2c,
2838c7a7, 4233d7e3, 5997b635). Session 5997 now detects 2 graphic regions
and removes 29 artifact words including "Tr" and "Su".

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

klausur-service/backend/cv_graphic_detect.py

@@ -170,7 +170,7 @@ def detect_graphic_elements(
             continue
 
         # Skip page-spanning regions
-        if bw > w * 0.5 or bh > h * 0.5:
+        if bw > w * 0.6 or bh > h * 0.6:
             logger.debug("GraphicDetect PASS1 skip page-spanning (%d,%d) %dx%d", bx, by, bw, bh)
             continue
 
@@ -232,12 +232,16 @@ def detect_graphic_elements(
         if color_pixel_count < 200:
             continue
 
-        # (d) Very low density → thin strokes, almost certainly text
-        if density < 0.20:
+        # (d) Very low density → thin strokes, almost certainly text.
+        # Large regions (photos/illustrations) can have low color density
+        # because most pixels are grayscale ink.  Use a lower threshold
+        # for regions bigger than 100×80 px.
+        _min_density = 0.05 if (bw > 100 and bh > 80) else 0.20
+        if density < _min_density:
             logger.info(
                 "GraphicDetect PASS1 skip low-density (%d,%d) %dx%d "
-                "density=%.0f%% (likely colored text)",
-                bx, by, bw, bh, density * 100,
+                "density=%.0f%% (min=%.0f%%, likely colored text)",
+                bx, by, bw, bh, density * 100, _min_density * 100,
             )
             continue