Fix ghost filter for borderless boxes + remove oversized graphic artifacts

1. Skip ghost filtering for boxes with border_thickness=0 (images/graphics
   have no border lines to produce OCR artifacts like |, I)
2. Remove individual word_boxes with height > 3x zone median (OCR from
   graphics like a huge "N" from a map image below text)

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

klausur-service/backend/tests/test_grid_editor_api.py

@@ -384,8 +384,8 @@ class TestFilterBorderGhosts:
         assert filtered[0]["text"] == "hello"
 
     def test_multi_char_ghost_kept(self):
-        """Multi-char '(=' on a box border → NOT filtered (real content)."""
-        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=0)
+        """Multi-char '(=' on a bordered box → NOT filtered (real content)."""
+        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=3)
         words = [
             {"text": "(=", "left": 644, "top": 294, "width": 16, "height": 17},
             {"text": "I", "left": 665, "top": 294, "width": 9, "height": 18},
@@ -394,6 +394,17 @@ class TestFilterBorderGhosts:
         assert count == 0
         assert len(filtered) == 2
 
+    def test_borderless_box_no_ghost_filter(self):
+        """Borderless box (border_thickness=0) → no ghost filtering at all."""
+        box = DetectedBox(x=648, y=129, width=338, height=125, confidence=0.7, border_thickness=0)
+        words = [
+            {"text": "I", "left": 643, "top": 272, "width": 6, "height": 19},  # near box edge
+            {"text": "|", "left": 647, "top": 200, "width": 3, "height": 10},  # even pipe on edge
+        ]
+        filtered, count = _filter_border_ghosts(words, [box])
+        assert count == 0  # nothing filtered — borderless box
+        assert len(filtered) == 2
+
     def test_single_paren_on_border_removed(self):
         """Single ')' on border → filtered."""
         box = DetectedBox(x=100, y=100, width=200, height=150, confidence=0.9, border_thickness=2)