fix: run color annotation on final cell word_boxes, not pre-grid words

_build_cells() creates new word_box dicts, so color fields set before
grid building were lost. Now detect_word_colors() runs after cells
are built, on the final word_boxes. Recovery still runs before grid
building so recovered words participate in column/row detection.

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

klausur-service/backend/grid_editor_api.py

@@ -450,10 +450,7 @@ async def build_grid(session_id: str):
         img_bgr = cv2.imdecode(arr, cv2.IMREAD_COLOR)
 
         if img_bgr is not None:
-            # --- Color detection: annotate existing words ---
-            detect_word_colors(img_bgr, all_words)
-
-            # --- Recover colored text that OCR missed ---
+            # --- Recover colored text that OCR missed (before grid building) ---
             recovered = recover_colored_text(img_bgr, all_words)
             if recovered:
                 recovered_count = len(recovered)
@@ -538,9 +535,17 @@ async def build_grid(session_id: str):
             **grid,
         })
 
+    # 5. Color annotation on final word_boxes in cells
+    if img_bgr is not None:
+        all_wb: List[Dict] = []
+        for z in zones_data:
+            for cell in z.get("cells", []):
+                all_wb.extend(cell.get("word_boxes", []))
+        detect_word_colors(img_bgr, all_wb)
+
     duration = time.time() - t0
 
-    # 5. Build result
+    # 6. Build result
     total_cells = sum(len(z.get("cells", [])) for z in zones_data)
     total_columns = sum(len(z.get("columns", [])) for z in zones_data)
     total_rows = sum(len(z.get("rows", [])) for z in zones_data)
@@ -575,7 +580,7 @@ async def build_grid(session_id: str):
         "duration_seconds": round(duration, 2),
     }
 
-    # 6. Persist to DB
+    # 7. Persist to DB
     await update_session_db(session_id, grid_editor_result=result)
 
     logger.info(