feat(ocr-pipeline): distinguish header/footer vs margin_top/margin_bottom

Check for actual ink content in detected top/bottom regions:
- 'header'/'footer' when text is present (e.g. title, page number)
- 'margin_top'/'margin_bottom' when the region is empty page margin

Also update all skip-type sets and color maps for the new types.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -840,6 +840,8 @@ async def _get_columns_overlay(session_id: str) -> Response:
         "column_ignore": (180, 180, 180), # Light Gray
         "header": (128, 128, 128),        # Gray
         "footer": (128, 128, 128),        # Gray
+        "margin_top": (100, 100, 100),    # Dark Gray
+        "margin_bottom": (100, 100, 100), # Dark Gray
     }
 
     overlay = img.copy()
@@ -1226,7 +1228,7 @@ async def _word_stream_generator(
 
     # Compute grid shape upfront for the meta event
     n_content_rows = len([r for r in row_geoms if r.row_type == 'content'])
-    _skip_types = {'column_ignore', 'header', 'footer', 'page_ref'}
+    _skip_types = {'column_ignore', 'header', 'footer', 'margin_top', 'margin_bottom', 'page_ref'}
     n_cols = len([c for c in col_regions if c.type not in _skip_types])
 
     # Determine layout
@@ -1712,6 +1714,8 @@ async def _get_rows_overlay(session_id: str) -> Response:
         "content": (255, 180, 0),     # Blue
         "header": (128, 128, 128),    # Gray
         "footer": (128, 128, 128),    # Gray
+        "margin_top": (100, 100, 100),    # Dark Gray
+        "margin_bottom": (100, 100, 100), # Dark Gray
     }
 
     overlay = img.copy()