Add Mailpit SMTP test server for Notification Service email testing

Adds a fourth Docker service (Mailpit) to capture outgoing emails without
delivery, with CLI tool for sending test emails, listing/reading captured
messages, and clearing the inbox. Supports BCC pattern matching ScadaLink's
notification delivery model.

infra/docker-compose.yml

@@ -41,5 +41,17 @@ services:
       - ./mssql/setup.sql:/docker-entrypoint-initdb.d/setup.sql:ro
     restart: unless-stopped
 
+  smtp:
+    image: axllent/mailpit:latest
+    container_name: scadalink-smtp
+    ports:
+      - "1025:1025"
+      - "8025:8025"
+    environment:
+      MP_SMTP_AUTH_ACCEPT_ANY: 1
+      MP_SMTP_AUTH_ALLOW_INSECURE: 1
+      MP_MAX_MESSAGES: 500
+    restart: unless-stopped
+
 volumes:
   scadalink-mssql-data: