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/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."""