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.Commons/Interfaces/Services/IInstanceLocator.cs

@@ -0,0 +1,16 @@
+namespace ScadaLink.Commons.Interfaces.Services;
+
+/// <summary>
+/// Resolves an instance unique name to its site identifier.
+/// Used by Inbound API's Route.To() to determine which site to route requests to.
+/// </summary>
+public interface IInstanceLocator
+{
+    /// <summary>
+    /// Resolves the site identifier for a given instance unique name.
+    /// Returns null if the instance is not found.
+    /// </summary>
+    Task<string?> GetSiteIdForInstanceAsync(
+        string instanceUniqueName,
+        CancellationToken cancellationToken = default);
+}