refactor: updated swagger;

feat: helper function in errors for checking postgres error types;
feat: sql query method for finding users by their email;
feat: registration begin/complete with checking existing username/email;
refactor: error handling in controller

sqlc/query.sql

@@ -34,6 +34,11 @@ WHERE id = $1;
 SELECT * FROM users 
 WHERE username = $1;
 
+;-- name: GetUserByEmail :one
+SELECT users.* FROM users
+JOIN login_informations linfo ON linfo.user_id = users.id
+WHERE linfo.email = @email::text;
+
 ;-- name: GetUserByLoginCredentials :one
 SELECT
   users.id,