feat: a2ui interface to enablee the LLM to give structured information

migrations/20260727120000_add_a2ui_messages/down.sql

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+DROP TABLE a2ui_messages;

migrations/20260727120000_add_a2ui_messages/up.sql

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+CREATE TABLE a2ui_messages (
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    session_id INTEGER NOT NULL,
+    message_id INTEGER NOT NULL,
+    json TEXT NOT NULL,
+    FOREIGN KEY (session_id) REFERENCES sessions(id),
+    FOREIGN KEY (message_id) REFERENCES messages(id)
+);
+
+CREATE INDEX a2ui_messages_session_id ON a2ui_messages(session_id, id);