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>

state.py

@@ -28,6 +28,7 @@ class AppState:
     _main_rendered: bool = False
     _live_checkboxes: dict = field(default_factory=dict)
     _live_refreshables: dict = field(default_factory=dict)
+    _src_cache: dict = field(default_factory=lambda: {'data': None, 'batch': [], 'name': None})
 
     def create_secondary(self) -> 'AppState':
         return AppState(