feat(backend): build backend with Nix and add CI

backend/settings/base.yaml

@@ -2,16 +2,6 @@ application:
   port: 3100
   version: "0.1.0"
 
-email:
-  host: email.example.com
-  port: 587
-  user: user
-  from: Contact Form <noreply@example.com>
-  password: hunter2
-  recipient: Admin <user@example.com>
-  starttls: false
-  tls: false
-
 rate_limit:
   enabled: true
   burst_size: 10