Add task_activity table schema

CREATE TABLE task_activity ( id INTEGER PRIMARY KEY AUTOINCREMENT, task_id TEXT NOT NULL, timestamp DATETIME DEFAULT CURRENT_TIMESTAMP, stage TEXT NOT NULL, -- claiming, running, reviewing, retrying, completed, failed message TEXT, metadata TEXT, -- JSON for extra data like retry count, error details FOREIGN KEY (task_id) REFERENCES tasks(id) )

Add to initTaskDb, create Haskell types, FromRow/ToRow instances