feat(api): expose /health endpoint and health dashboard

Wire the infrastructure health checks into ASP.NET Core:

- /health returns JSON via UIResponseWriter and maps Unhealthy to HTTP 503
- /health-ui renders a Razor Pages dashboard with per-check status, description, tags and duration
- Tag health checks (rabbitmq, workers, ready, live) for monitoring filters
- Link the dashboard from the Swagger description and the page layout

src/presentation/DigitalData.MessagingService.API/Pages/_Layout.cshtml

@@ -0,0 +1,28 @@
+<!DOCTYPE html>
+<html lang="de">
+<head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>@ViewData["Title"] — DigitalData MessagingService</title>
+    <style>
+        *, *::before, *::after { box-sizing: border-box; }
+        body { margin: 0; font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; background: #0f1117; color: #e2e8f0; }
+        header { background: #1a1d2e; border-bottom: 1px solid #2d3148; padding: 0 2rem; display: flex; align-items: center; height: 56px; gap: 1.5rem; }
+        header a { color: #a78bfa; text-decoration: none; font-weight: 600; font-size: 1rem; }
+        header a:hover { color: #c4b5fd; }
+        header .brand { color: #e2e8f0; font-size: 1.1rem; font-weight: 700; margin-right: auto; }
+        main { padding: 2rem; max-width: 1100px; margin: 0 auto; }
+    </style>
+</head>
+<body>
+    <header>
+        <span class="brand">DigitalData MessagingService</span>
+        <a href="/health-ui">Health</a>
+        <a href="/serilog-ui">Logs</a>
+        <a href="/swagger">API</a>
+    </header>
+    <main>
+        @RenderBody()
+    </main>
+</body>
+</html>