Fix blocking I/O on event loop, cache graphviz, optimize DB sync

Move all save_json/load_json/sync_to_db/DB calls off the event loop
with asyncio.to_thread to prevent UI freezes. Cache graphviz SVG by
DOT source hash (bounded LRU of 20). Replace DELETE-all/re-INSERT in
sync_to_db with UPSERT + targeted DELETE. Add DB indexes, COUNT query,
and reduce graph poll interval to 0.5s.

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

history_tree.py

@@ -129,9 +129,10 @@ class HistoryTree:
             while current and current in self.nodes:
                 if current in visited:
                     break
+                if current in node_to_branch:
+                    break  # this node and all ancestors already assigned
                 visited.add(current)
-                if current not in node_to_branch:
-                    node_to_branch[current] = b_name
+                node_to_branch[current] = b_name
                 current = self.nodes[current].get('parent')
 
         # Per-branch color palette (bg, border) — cycles for many branches