Query
CREATE TABLE credit_transactions (
id INTEGER PRIMARY KEY AUTOINCREMENT,
user_id INTEGER NOT NULL,
chatbot_id INTEGER,
amount_delta INTEGER NOT NULL,
reason TEXT NOT NULL,
reference_id TEXT NOT NULL UNIQUE,
created_at TEXT NOT NULL,
FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
FOREIGN KEY (chatbot_id) REFERENCES chatbots(id) ON DELETE SET NULL
)