add: register and login with DB query AND some struct type correction

test/http/shortlink-flow.http

@@ -7,8 +7,8 @@ POST http://localhost:5555/auth/login
 Content-Type: application/json
 
 {
-  "email": "admin",
-  "password": "admin"
+  "email": "patient1@example.com",
+  "password": "password123"
 }