-- Whether the user can receive messages from the bot (false after block / 403; true after /start or my_chat_member member).
ALTER TABLE db_users_a
    ADD COLUMN bot_active TINYINT(1) NOT NULL DEFAULT 1 COMMENT '1 = bot can message user, 0 = blocked/unreachable';

-- Composite index: COUNT(bot_active=0), and id-range queries with bot_active=true (notification broadcast).
CREATE INDEX idx_users_a_bot_active_id ON db_users_a(bot_active, id);
