chore: add per-cluster debug logging for column alignment detection

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

klausur-service/backend/grid_editor_api.py

@@ -165,6 +165,12 @@ def _cluster_columns_by_alignment(
     ]
     significant = sorted(primary + secondary, key=lambda c: c["mean_x"])
 
+    for c in significant:
+        logger.info(
+            "  significant cluster: x=%d (range %d-%d), %d words, %d rows (%.0f%%)",
+            c["mean_x"], c["min_edge"], c["max_edge"],
+            c["count"], c["distinct_rows"], c["row_coverage"] * 100,
+        )
     logger.info(
         "alignment columns: %d clusters, %d primary, %d secondary → %d significant",
         len(clusters), len(primary), len(secondary), len(significant),