Add A/B testing toggles to OCR Kombi Grid Review

Quality step toggles in admin-lehrer StepGridReview (port 3002):
- CLAHE checkbox (Step 3: image enhancement)
- MaxCol dropdown (Step 2: column limit, 0=off)
- MinConf dropdown (Step 1: OCR confidence, 0=auto)

Parameters flow through: StepGridReview → useGridEditor → build-grid
endpoint → _build_grid_core. MinConf filters words before grid building.

Toggle settings, click "Neu berechnen" to test each step individually.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/grid_build_core.py

@@ -55,6 +55,9 @@ async def _build_grid_core(
     *,
     ipa_mode: str = "auto",
     syllable_mode: str = "auto",
+    enhance: bool = True,
+    max_columns: int | None = None,
+    min_conf: int | None = None,
 ) -> dict:
     """Core grid building logic — pure computation, no HTTP or DB side effects.
 
@@ -91,8 +94,18 @@ async def _build_grid_core(
     if not all_words:
         raise ValueError("No word boxes found in cells")
 
-    logger.info("build-grid session %s: %d words from %d cells",
-                session_id, len(all_words), len(word_result["cells"]))
+    # 2a-pre. Apply min_conf filter if specified (Step 1: confidence tightening)
+    if min_conf and min_conf > 0:
+        before = len(all_words)
+        all_words = [w for w in all_words if w.get('conf', 100) >= min_conf]
+        removed = before - len(all_words)
+        if removed:
+            logger.info("build-grid session %s: min_conf=%d removed %d/%d words",
+                        session_id, min_conf, removed, before)
+
+    logger.info("build-grid session %s: %d words from %d cells (enhance=%s, max_cols=%s, min_conf=%s)",
+                session_id, len(all_words), len(word_result["cells"]),
+                enhance, max_columns, min_conf)
 
     # 2b. Filter decorative margin columns (alphabet graphics).
     # Some worksheets have a decorative alphabet strip along one margin