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.

test_infra.md

@@ -1,6 +1,6 @@
 # Test Infrastructure
 
-This document describes the local Docker-based test infrastructure for ScadaLink development. Three services provide the external dependencies needed to run and test the system locally.
+This document describes the local Docker-based test infrastructure for ScadaLink development. Four services provide the external dependencies needed to run and test the system locally.
 
 ## Services
 
@@ -9,6 +9,7 @@ This document describes the local Docker-based test infrastructure for ScadaLink
 | OPC UA Server | `mcr.microsoft.com/iotedge/opc-plc:latest` | 50000 (OPC UA), 8080 (web) | `infra/opcua/nodes.json` |
 | LDAP Server | `glauth/glauth:latest` | 3893 | `infra/glauth/config.toml` |
 | MS SQL 2022 | `mcr.microsoft.com/mssql/server:2022-latest` | 1433 | `infra/mssql/setup.sql` |
+| SMTP (Mailpit) | `axllent/mailpit:latest` | 1025 (SMTP), 8025 (web) | Environment vars |
 
 ## Quick Start
 
@@ -32,6 +33,7 @@ Each service has a dedicated document with configuration details, verification s
 - [test_infra_opcua.md](test_infra_opcua.md) — OPC UA test server (Azure IoT OPC PLC)
 - [test_infra_ldap.md](test_infra_ldap.md) — LDAP test server (GLAuth)
 - [test_infra_db.md](test_infra_db.md) — MS SQL 2022 database
+- [test_infra_smtp.md](test_infra_smtp.md) — SMTP test server (Mailpit)
 
 ## Connection Strings
 
@@ -51,6 +53,13 @@ For use in `appsettings.Development.json`:
   },
   "OpcUa": {
     "EndpointUrl": "opc.tcp://localhost:50000"
+  },
+  "Smtp": {
+    "Server": "localhost",
+    "Port": 1025,
+    "AuthMode": "None",
+    "FromAddress": "scada-notifications@company.com",
+    "ConnectionTimeout": 30
   }
 }
 ```
@@ -60,7 +69,7 @@ For use in `appsettings.Development.json`:
 ```bash
 cd infra
 docker compose down            # stop containers, preserve SQL data volume
-docker compose stop opcua      # stop a single service (also: ldap, mssql)
+docker compose stop opcua      # stop a single service (also: ldap, mssql, smtp)
 ```
 
 **Full teardown** (removes volumes, optionally images and venv):
@@ -77,11 +86,11 @@ After a full teardown, the next `docker compose up -d` starts fresh — re-run t
 
 ```
 infra/
-  docker-compose.yml          # All three services
+  docker-compose.yml          # All four services
   teardown.sh                 # Teardown script (volumes, images, venv)
   glauth/config.toml          # LDAP users and groups
   mssql/setup.sql             # Database and user creation
   opcua/nodes.json            # Custom OPC UA tag definitions
-  tools/                      # Python CLI tools (opcua, ldap, mssql)
+  tools/                      # Python CLI tools (opcua, ldap, mssql, smtp)
   README.md                   # Quick-start for the infra folder
 ```