fix: lower tertiary gap threshold for narrow margin column detection

Reduce gap threshold from max(40, 5%) to max(30, 2%) so page_ref
columns (e.g. p.55/p.57) at ~56px gap are detected as tertiary columns.

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

klausur-service/backend/grid_editor_api.py

@@ -188,7 +188,7 @@ def _cluster_columns_by_alignment(
         # Must have significant gap to nearest significant cluster
         if sig_xs:
             min_dist = min(abs(c["mean_x"] - sx) for sx in sig_xs)
-            if min_dist < max(40, content_span * 0.05):
+            if min_dist < max(30, content_span * 0.02):
                 continue
         tertiary.append(c)