fix(ocr-pipeline): dewarp visibility, grid on both sides, session persistence

- Fix dewarp method selection: prefer methods with >5px curvature over
  higher confidence (vertical_edge 79px was being ignored for text_baseline 2px)
- Add grid overlay on left image in Dewarp step for side-by-side comparison
- Add GET /sessions/{id} endpoint to reload session data
- StepDeskew accepts sessionId prop to restore state when navigating back
- SessionInfo type extended with optional deskew_result and dewarp_result

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -627,12 +627,24 @@ def dewarp_image(img: np.ndarray) -> Tuple[np.ndarray, Dict[str, Any]]:
                 f"curv={result_b['curvature_px']:.1f}px "
                 f"({duration:.2f}s)")
 
-    # Pick method with higher confidence
-    if result_a["confidence"] >= result_b["confidence"]:
+    # Pick best method: prefer significant curvature over high confidence
+    # If one method found real curvature (>5px) and the other didn't (<3px),
+    # prefer the one with real curvature regardless of confidence.
+    a_has_curvature = result_a["curvature_px"] >= 5.0 and result_a["displacement_map"] is not None
+    b_has_curvature = result_b["curvature_px"] >= 5.0 and result_b["displacement_map"] is not None
+
+    if a_has_curvature and not b_has_curvature:
+        best = result_a
+    elif b_has_curvature and not a_has_curvature:
+        best = result_b
+    elif result_a["confidence"] >= result_b["confidence"]:
         best = result_a
     else:
         best = result_b
 
+    logger.info(f"dewarp: selected {best['method']} "
+                f"(curv={best['curvature_px']:.1f}px, conf={best['confidence']:.2f})")
+
     if best["displacement_map"] is None or best["curvature_px"] < 2.0:
         return img, no_correction