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/README.md

@@ -8,13 +8,14 @@ Local Docker-based test services for ScadaLink development.
 docker compose up -d
 ```
 
-This starts three services:
+This starts four services:
 
 | Service | Port | Purpose |
 |---------|------|---------|
 | OPC UA (Azure IoT OPC PLC) | 50000 (OPC UA), 8080 (web) | Simulated OPC UA server with ScadaLink-style tags |
 | LDAP (GLAuth) | 3893 | Lightweight LDAP with test users/groups matching ScadaLink roles |
 | MS SQL 2022 | 1433 | Configuration and machine data databases |
+| SMTP (Mailpit) | 1025 (SMTP), 8025 (web) | Email capture for notification testing |
 
 ## First-Time SQL Setup
 
@@ -37,7 +38,7 @@ docker compose down
 
 **Stop a single service** (leave the others running):
 ```bash
-docker compose stop opcua      # or: ldap, mssql
+docker compose stop opcua      # or: ldap, mssql, smtp
 docker compose start opcua     # bring it back without recreating
 ```
 
@@ -70,11 +71,12 @@ pip install -r tools/requirements.txt
 
 > The `.venv` directory is gitignored.
 
-**Quick readiness check** (all three services, with venv active):
+**Quick readiness check** (all four services, with venv active):
 ```bash
 python tools/opcua_tool.py check
 python tools/ldap_tool.py check
 python tools/mssql_tool.py check
+python tools/smtp_tool.py check
 ```
 
 | Tool | Service | Key Commands |
@@ -82,6 +84,7 @@ python tools/mssql_tool.py check
 | `tools/opcua_tool.py` | OPC UA | `check`, `browse`, `read`, `write`, `monitor` |
 | `tools/ldap_tool.py` | LDAP | `check`, `bind`, `search`, `users`, `groups` |
 | `tools/mssql_tool.py` | MS SQL | `check`, `setup`, `query`, `tables` |
+| `tools/smtp_tool.py` | SMTP (Mailpit) | `check`, `send`, `list`, `read`, `clear` |
 
 Each tool supports `--help` for full usage. See the per-service docs below for detailed examples.
 
@@ -93,3 +96,4 @@ See the project root for per-service setup guides:
 - [test_infra_opcua.md](../test_infra_opcua.md) — OPC UA server details
 - [test_infra_ldap.md](../test_infra_ldap.md) — LDAP server details
 - [test_infra_db.md](../test_infra_db.md) — MS SQL database details
+- [test_infra_smtp.md](../test_infra_smtp.md) — SMTP server details (Mailpit)