How to allow users to log on only from my application not from pgadmin

My application implements field and row level security.
I have custom table of users where user privileges are described.

However user can login directly to database using pgAdmin. This bypasses
the security.

How to allow users to login only from my application ?
I think I must create server-side pgsql procedure for login validation.

How to implement custom authentication in server side ?

How to force PostgreSQL to call stored procedure for user logon validation
when user logins to PostgreSQL ?

Andurs.