Phase 8: Production readiness — failover tests, security hardening, sandboxing, deployment docs

- WP-1-3: Central/site failover + dual-node recovery tests (17 tests)
- WP-4: Performance testing framework for target scale (7 tests)
- WP-5: Security hardening (LDAPS, JWT key length, no secrets in logs) (11 tests)
- WP-6: Script sandboxing adversarial tests (28 tests, all forbidden APIs)
- WP-7: Recovery drill test scaffolds (5 tests)
- WP-8: Observability validation (structured logs, correlation IDs, metrics) (6 tests)
- WP-9: Message contract compatibility (forward/backward compat) (18 tests)
- WP-10: Deployment packaging (installation guide, production checklist, topology)
- WP-11: Operational runbooks (failover, troubleshooting, maintenance)
92 new tests, all passing. Zero warnings.

src/ScadaLink.NotificationService/ISmtpClientWrapper.cs

@@ -0,0 +1,12 @@
+namespace ScadaLink.NotificationService;
+
+/// <summary>
+/// Abstraction over SMTP client for testability.
+/// </summary>
+public interface ISmtpClientWrapper
+{
+    Task ConnectAsync(string host, int port, bool useTls, CancellationToken cancellationToken = default);
+    Task AuthenticateAsync(string authType, string? credentials, CancellationToken cancellationToken = default);
+    Task SendAsync(string from, IEnumerable<string> bccRecipients, string subject, string body, CancellationToken cancellationToken = default);
+    Task DisconnectAsync(CancellationToken cancellationToken = default);
+}