fix: Word-Erkennung ohne Spalten ermoeglichen (Full-Page Pseudo-Column)

Wenn column_result fehlt (z.B. OCR Overlay Pipeline), wird automatisch
eine einzelne ganzseitige Pseudo-Spalte erzeugt statt einen Fehler zu werfen.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -1883,7 +1883,21 @@ async def detect_words(
     column_result = session.get("column_result")
     row_result = session.get("row_result")
     if not column_result or not column_result.get("columns"):
-        raise HTTPException(status_code=400, detail="Column detection must be completed first")
+        # No column detection — synthesize a single full-page pseudo-column.
+        # This enables the overlay pipeline which skips column detection.
+        img_h_tmp, img_w_tmp = dewarped_bgr.shape[:2]
+        column_result = {
+            "columns": [{
+                "type": "column_text",
+                "x": 0, "y": 0,
+                "width": img_w_tmp, "height": img_h_tmp,
+                "classification_confidence": 1.0,
+                "classification_method": "full_page_fallback",
+            }],
+            "zones": [],
+            "duration_seconds": 0,
+        }
+        logger.info("detect_words: no column_result — using full-page pseudo-column %dx%d", img_w_tmp, img_h_tmp)
     if not row_result or not row_result.get("rows"):
         raise HTTPException(status_code=400, detail="Row detection must be completed first")