How to ensure a log-entry is created based on state of data in other tables

DROP TABLE IF EXISTS activity_product_log;
DROP TABLE IF EXISTS activity;
DROP TABLE IF EXISTS product;

CREATE TABLE product
(
    id INTEGER primary key,
    name varchar not null unique
);

CREATE TABLE activity
(
    id serial primary key,
    name varchar not null unique,
    product_id integer not null references product(id),
    status varchar NOT NULL,
    CHECK (status IN ('NOT_DONE', 'DONE'))
);

CREATE TABLE activity_product_log
(
    id serial primary key,
    product_id integer not null references product(id),
    created TIMESTAMP NOT NULL
);

INSERT INTO product(id, name) VALUES(1, 'Horse'), (2, 'Fish');

INSERT INTO activity(name, product_id, status)
VALUES ('A1', 1, 'NOT_DONE'), ('A2', 1, 'NOT_DONE'), ('A3', 1, 'NOT_DONE'), ('A4', 1, 'NOT_DONE')
     , ('A5', 2, 'NOT_DONE'), ('A6', 2, 'NOT_DONE')
;

UPDATE activity SET status = 'DONE' WHERE name = 'A1';
UPDATE activity SET status = 'DONE' WHERE name = 'A2';
UPDATE activity SET status = 'DONE' WHERE name = 'A3';
UPDATE activity SET status = 'DONE' WHERE name = 'A4'; -- Here an entry in activity_log should be created