feat: group_node schema + projector handlers

chat/db/migrations/0008_group_node.sql

@@ -0,0 +1,8 @@
+CREATE TABLE group_node (
+    chat_id TEXT PRIMARY KEY,
+    members_json TEXT NOT NULL,
+    summary TEXT NOT NULL DEFAULT '',
+    dynamic TEXT NOT NULL DEFAULT '',
+    threads_json TEXT NOT NULL DEFAULT '[]',
+    updated_at TEXT NOT NULL DEFAULT (datetime('now'))
+);