Fix missing import, add transaction safety, clean orphaned snapshots

- Add load_json to tab_timeline_ng imports (NameError on disk fallback)
- Wrap save_history_tree in BEGIN/COMMIT transaction (was autocommitting
  each statement, risking partial writes on failure)
- Clean up orphaned history_snapshots in sync_to_db when nodes are
  removed from the tree

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

db.py

@@ -326,10 +326,16 @@ class ProjectDB:
     def save_history_tree(self, data_file_id: int, tree_data: dict) -> None:
         """Save history tree, extracting node snapshots into separate table."""
         now = time.time()
-        # Extract snapshot data from nodes into history_snapshots table
         nodes = tree_data.get("nodes", {})
         slim_tree = dict(tree_data)
         slim_nodes = {}
+        for nid, node in nodes.items():
+            slim_nodes[nid] = {k: v for k, v in node.items() if k != "data"}
+        slim_tree["nodes"] = slim_nodes
+
+        self.conn.execute("BEGIN IMMEDIATE")
+        try:
+            # Extract snapshot data from nodes into history_snapshots table
             for nid, node in nodes.items():
                 snap = node.get("data")
                 if snap:
@@ -340,16 +346,19 @@ class ProjectDB:
                         "snapshot_data=excluded.snapshot_data, updated_at=excluded.updated_at",
                         (data_file_id, nid, json.dumps(snap), now),
                     )
-            # Store node without data in tree
-            slim_nodes[nid] = {k: v for k, v in node.items() if k != "data"}
-        slim_tree["nodes"] = slim_nodes
             self.conn.execute(
                 "INSERT INTO history_trees (data_file_id, tree_data, updated_at) "
                 "VALUES (?, ?, ?) "
                 "ON CONFLICT(data_file_id) DO UPDATE SET tree_data=excluded.tree_data, updated_at=excluded.updated_at",
                 (data_file_id, json.dumps(slim_tree), now),
             )
-        self.conn.commit()
+            self.conn.execute("COMMIT")
+        except Exception:
+            try:
+                self.conn.execute("ROLLBACK")
+            except Exception:
+                pass
+            raise
 
     def get_history_tree(self, data_file_id: int) -> dict | None:
         """Load history tree metadata (without snapshot data)."""

tab_timeline_ng.py

@@ -8,7 +8,7 @@ from nicegui import ui
 
 from state import AppState
 from history_tree import HistoryTree
-from utils import save_json, sync_to_db, KEY_BATCH_DATA, KEY_HISTORY_TREE
+from utils import save_json, load_json, sync_to_db, KEY_BATCH_DATA, KEY_HISTORY_TREE
 
 logger = logging.getLogger(__name__)
 
@@ -580,8 +580,7 @@ async def _restore_node(data, node, htree, file_path, state: AppState):
                 state.db.get_node_snapshot, df['id'], node['id'])
     if not raw_snap:
         # Last resort: read from JSON file on disk
-        from utils import load_json as _load_json
+        raw_file, _ = await asyncio.to_thread(load_json, file_path)
-        raw_file, _ = await asyncio.to_thread(_load_json, file_path)
         tree_on_disk = raw_file.get(KEY_HISTORY_TREE, {})
         raw_snap = tree_on_disk.get('nodes', {}).get(node['id'], {}).get('data', {})
     node_data = json.loads(json.dumps(raw_snap)) if raw_snap else {}

utils.py

@@ -312,6 +312,20 @@ def sync_to_db(db, project_name: str, file_path: Path, data: dict) -> None:
                     "ON CONFLICT(data_file_id) DO UPDATE SET tree_data=excluded.tree_data, updated_at=excluded.updated_at",
                     (df_id, json.dumps(slim_tree), now),
                 )
+                # Clean up orphaned snapshots for nodes no longer in tree
+                current_node_ids = set(nodes.keys())
+                if current_node_ids:
+                    placeholders = ",".join("?" for _ in current_node_ids)
+                    db.conn.execute(
+                        f"DELETE FROM history_snapshots WHERE data_file_id = ? "
+                        f"AND node_id NOT IN ({placeholders})",
+                        (df_id, *current_node_ids),
+                    )
+                else:
+                    db.conn.execute(
+                        "DELETE FROM history_snapshots WHERE data_file_id = ?",
+                        (df_id,),
+                    )
 
             db.conn.execute("COMMIT")
         except Exception: