guard: routes - POST /server/health/test-message

The owner, 2026-10-16: "Делай все согласованные" - test-email among them,
its sender his choice of the same day ("Да, адрес годится"); the form is
Health's card 09 (mockups e92769f, e3b2501) and posts to a route under the
page it belongs to, as Re-check DNS and Reload do. The guard's route table
gets the one pattern. Nothing else under internal/web changes in this
commit; the router and the handler follow in the next.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/guard_routes_test.go

@@ -23,7 +23,7 @@ var routes = struct{ always, inbound, dmarc []string }{
 		"GET /{$}",
 		"GET /overview", "GET /overview/fragment",
 		"GET /server/health", "GET /server/health/fragment",
-		"POST /server/health/recheck", "POST /server/health/reload",
+		"POST /server/health/recheck", "POST /server/health/reload", "POST /server/health/test-message",
 		"GET /outbound/domains", "POST /outbound/domains",
 		"GET /outbound/domains/{id}", "POST /outbound/domains/{id}/dns-recheck",
 		"GET /outbound/domains/{id}/settings",