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

backend/settings/development.yaml

@@ -12,5 +12,7 @@ email:
   port: 1025
   user: ""
   password: ""
+  from: Contact Form <noreply@example.com>
+  recipient: Admin <user@example.com>
   tls: false
   starttls: false

backend/settings/production.yaml

@@ -6,3 +6,13 @@ application:
   protocol: https
   host: 0.0.0.0
   base_url: ""
+
+email:
+  host: ""
+  port: 0
+  user: ""
+  password: ""
+  from: ""
+  recipient: ""
+  tls: false
+  starttls: false