Fix RAM leak: strip history snapshots from memory, load on demand

History tree nodes stored full data snapshots in memory (5-50MB each),
accumulating with every save. Now:

- New `history_snapshots` DB table stores node data separately
- `save_history_tree` and `sync_to_db` extract snapshots before saving
- In-memory tree nodes only hold metadata (id, parent, note, timestamp)
- Restore and preview load snapshots from DB on demand
- `save_and_snap` uses json roundtrip instead of deepcopy (1 copy not 2)
- `_src_cache` moved to AppState, cleared on file switch
- `strip_snapshots()` method on HistoryTree for explicit cleanup

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

history_tree.py

@@ -76,6 +76,11 @@ class HistoryTree:
             return self.nodes[node_id]["data"]
         return None
 
+    def strip_snapshots(self) -> None:
+        """Remove snapshot data from all nodes to free memory."""
+        for node in self.nodes.values():
+            node.pop("data", None)
+
     def to_dict(self) -> dict[str, Any]:
         return {"nodes": self.nodes, "branches": self.branches, "head_id": self.head_id}