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
 

klausur-service/backend/ocr_pipeline_api.py

@@ -149,6 +149,32 @@ async def create_session(file: UploadFile = File(...)):
     }
 
 
+@router.get("/sessions/{session_id}")
+async def get_session_info(session_id: str):
+    """Get session info including deskew/dewarp results for step navigation."""
+    session = _get_session(session_id)
+    img_bgr = session["original_bgr"]
+
+    result = {
+        "session_id": session["id"],
+        "filename": session["filename"],
+        "image_width": img_bgr.shape[1],
+        "image_height": img_bgr.shape[0],
+        "original_image_url": f"/api/v1/ocr-pipeline/sessions/{session_id}/image/original",
+        "current_step": session.get("current_step", 1),
+    }
+
+    # Include deskew result if available
+    if session.get("deskew_result"):
+        result["deskew_result"] = session["deskew_result"]
+
+    # Include dewarp result if available
+    if session.get("dewarp_result"):
+        result["dewarp_result"] = session["dewarp_result"]
+
+    return result
+
+
 @router.post("/sessions/{session_id}/deskew")
 async def auto_deskew(session_id: str):
     """Run both deskew methods and pick the best one."""