fix(ocr-pipeline): skip edge-touching gaps in header/footer detection

Gaps that extend to the image boundary (top/bottom edge) are not valid
content separators — they typically represent dewarp padding. Only gaps
with content on both sides qualify as header/footer boundaries.

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -2593,6 +2593,9 @@ def _detect_header_footer_gaps(
     large_gap_threshold = median_gap * GAP_MULTIPLIER
 
     # Step 6: Find largest qualifying gap in header / footer zones
+    # A separator gap must have content on BOTH sides — edge-touching gaps
+    # (e.g. dewarp padding at bottom) are not valid separators.
+    EDGE_MARGIN = max(5, actual_h // 400)
     header_zone_limit = int(actual_h * HEADER_FOOTER_ZONE)
     footer_zone_start = int(actual_h * (1.0 - HEADER_FOOTER_ZONE))
 
@@ -2601,6 +2604,8 @@ def _detect_header_footer_gaps(
 
     best_header_size = 0
     for gs, ge in raw_gaps:
+        if gs <= EDGE_MARGIN:
+            continue  # skip gaps touching the top edge
         gap_mid = (gs + ge) / 2
         gap_size = ge - gs
         if gap_mid < header_zone_limit and gap_size > large_gap_threshold:
@@ -2610,6 +2615,8 @@ def _detect_header_footer_gaps(
 
     best_footer_size = 0
     for gs, ge in raw_gaps:
+        if ge >= actual_h - EDGE_MARGIN:
+            continue  # skip gaps touching the bottom edge
         gap_mid = (gs + ge) / 2
         gap_size = ge - gs
         if gap_mid > footer_zone_start and gap_size > large_gap_threshold: