refactor: independent sessions for page-split + URL-based pipeline navigation

Page-split now creates independent sessions (no parent_session_id),
parent marked as status='split' and hidden from list. Navigation uses
useSearchParams for URL-based step tracking (browser back/forward works).
page.tsx reduced from 684 to 443 lines via usePipelineNavigation hook.

Box sub-sessions (column detection) remain unchanged.

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

klausur-service/backend/ocr_pipeline_session_store.py

@@ -238,7 +238,7 @@ async def list_sessions_db(
     """
     pool = await get_pool()
     async with pool.acquire() as conn:
-        where = "" if include_sub_sessions else "WHERE parent_session_id IS NULL"
+        where = "" if include_sub_sessions else "WHERE parent_session_id IS NULL AND (status IS NULL OR status != 'split')"
         rows = await conn.fetch(f"""
             SELECT id, name, filename, status, current_step,
                    document_category, doc_type,

klausur-service/backend/ocr_pipeline_sessions.py

@@ -191,12 +191,12 @@ async def get_session_info(session_id: str):
     if session.get("ground_truth"):
         result["ground_truth"] = session["ground_truth"]
 
-    # Sub-session info
+    # Box sub-session info (zone_type='box' from column detection — NOT page-split)
     if session.get("parent_session_id"):
         result["parent_session_id"] = session["parent_session_id"]
         result["box_index"] = session.get("box_index")
     else:
-        # Check for sub-sessions
+        # Check for box sub-sessions (column detection creates these)
         subs = await get_sub_sessions(session_id)
         if subs:
             result["sub_sessions"] = [

klausur-service/backend/orientation_crop_api.py

@@ -238,8 +238,8 @@ async def detect_page_split(session_id: str):
         "duration_seconds": round(duration, 2),
     }
 
-    # Mark parent session as split (store info in crop_result for backward compat)
-    await update_session_db(session_id, crop_result=split_info)
+    # Mark parent session as split and hidden from session list
+    await update_session_db(session_id, crop_result=split_info, status='split')
     cached["crop_result"] = split_info
 
     await _append_pipeline_log(session_id, "page_split", {
@@ -346,6 +346,7 @@ async def auto_crop(session_id: str):
             cropped_png=png_buf.tobytes() if ok else b"",
             crop_result=crop_info,
             current_step=5,
+            status='split',
         )
 
         logger.info(
@@ -461,8 +462,6 @@ async def _create_page_sub_sessions(
             name=sub_name,
             filename=parent_filename,
             original_png=page_png,
-            parent_session_id=parent_session_id,
-            box_index=pi,
         )
 
         # Pre-populate: set cropped = original (already cropped)
@@ -540,8 +539,6 @@ async def _create_page_sub_sessions_full(
             name=sub_name,
             filename=parent_filename,
             original_png=page_png,
-            parent_session_id=parent_session_id,
-            box_index=pi,
         )
 
         # start_step=2 → ready for deskew (orientation already done on spread)
@@ -553,7 +550,6 @@ async def _create_page_sub_sessions_full(
             "id": sub_id,
             "filename": parent_filename,
             "name": sub_name,
-            "parent_session_id": parent_session_id,
             "original_bgr": page_bgr,
             "oriented_bgr": None,
             "cropped_bgr": None,