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:

klausur-service/backend/tests/test_cv_vocab_pipeline.py

@@ -1092,6 +1092,17 @@ class TestHeaderFooterGapDetection:
         assert header_y is None
         assert footer_y is None
 
+    def test_edge_gaps_ignored_dewarp_padding(self):
+        """Trailing gap at bottom edge (dewarp padding) should not be detected as footer."""
+        h, w = 2000, 800
+        # Body lines from 10 to 1700
+        bands = self._make_body_with_lines(h, w, 10, 1700)
+        # Gap from 1700 to 2000 = bottom edge padding (no content after)
+        inv = self._make_inv(h, w, bands)
+        header_y, footer_y = _detect_header_footer_gaps(inv, w, h)
+        # The trailing gap touches the image edge → not a valid separator
+        assert footer_y is None
+
 
 class TestRegionContentCheck:
     """Tests for _region_has_content() and _add_header_footer() type selection."""