Constraint via Foreign Key
You only need some small changes. First add a UNIQUE constraint on calendars_entry_types (this is needed for the new foreign key below):

UNIQUE (calendar_id, entry_type_id)


Then remove these two foreign keys from entries:

FOREIGN KEY (calendar_id) REFERENCES calendars (id), 
FOREIGN KEY (entry_type_id) REFERENCES entry_types (id), 


and replace them with a composite one:

FOREIGN KEY (calendar_id, entry_type_id)
    REFERENCES calendars_entry_types (calendar_id, entry_type_id),