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/DigitalData.MessagingService.API.csproj

@@ -26,8 +26,9 @@
     <PackageReference Include="Serilog.UI" Version="3.2.0" />
     <PackageReference Include="Serilog.UI.SqliteProvider" Version="1.1.0" />
     <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />
-	<PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.10" />
-	<PackageReference Include="System.Security.Cryptography.Xml" Version="10.0.10" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.10" />
+    <PackageReference Include="System.Security.Cryptography.Xml" Version="10.0.10" />
+    <PackageReference Include="AspNetCore.HealthChecks.UI.Client" Version="8.0.1" />
   </ItemGroup>
 
   <ItemGroup>

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

@@ -0,0 +1,68 @@
+@page "/health-ui"
+@model DigitalData.MessagingService.API.Pages.HealthDashboardModel
+@{
+    ViewData["Title"] = "Health Dashboard";
+    Layout = "_Layout";
+}
+
+<style>
+    h1 { font-size: 1.5rem; margin-bottom: 1.5rem; color: #e2e8f0; }
+    .overall { display: inline-flex; align-items: center; gap: .5rem; font-size: 1rem; font-weight: 600;
+               padding: .35rem .9rem; border-radius: 999px; margin-bottom: 2rem; }
+    .overall.Healthy   { background: #14532d; color: #4ade80; }
+    .overall.Degraded  { background: #713f12; color: #fbbf24; }
+    .overall.Unhealthy { background: #7f1d1d; color: #f87171; }
+    .grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(300px, 1fr)); gap: 1rem; }
+    .card { background: #1a1d2e; border: 1px solid #2d3148; border-radius: .75rem; padding: 1.25rem; }
+    .card-header { display: flex; justify-content: space-between; align-items: center; margin-bottom: .75rem; }
+    .card-name { font-weight: 600; font-size: .95rem; }
+    .badge { font-size: .75rem; font-weight: 700; padding: .2rem .6rem; border-radius: 999px; }
+    .badge-Healthy   { background: #14532d; color: #4ade80; }
+    .badge-Degraded  { background: #713f12; color: #fbbf24; }
+    .badge-Unhealthy { background: #7f1d1d; color: #f87171; }
+    .desc { font-size: .82rem; color: #94a3b8; }
+    .tags { margin-top: .6rem; display: flex; gap: .4rem; flex-wrap: wrap; }
+    .tag { font-size: .7rem; background: #2d3148; color: #94a3b8; padding: .15rem .5rem; border-radius: 4px; }
+    .duration { font-size: .75rem; color: #64748b; margin-top: .5rem; }
+    .refresh { font-size: .8rem; color: #64748b; margin-bottom: 1.5rem; }
+    .refresh a { color: #a78bfa; text-decoration: none; }
+</style>
+
+<h1>Health Dashboard</h1>
+
+<div class="overall @Model.OverallStatus.ToString()">
+    <span>@(Model.OverallStatus == Microsoft.Extensions.Diagnostics.HealthChecks.HealthStatus.Healthy ? "?" : "?")</span>
+    <span>@Model.OverallStatus</span>
+</div>
+
+<p class="refresh">
+    Zuletzt geprüft: <strong>@Model.CheckedAt.ToString("yyyy-MM-dd HH:mm:ss")</strong> UTC
+    &nbsp;·&nbsp; <a href="/health-ui">Aktualisieren</a>
+    &nbsp;·&nbsp; <a href="/health" target="_blank">JSON</a>
+</p>
+
+<div class="grid">
+    @foreach (var entry in Model.Entries)
+    {
+        <div class="card">
+            <div class="card-header">
+                <span class="card-name">@entry.Name</span>
+                <span class="badge badge-@entry.Status.ToString()">@entry.Status</span>
+            </div>
+            @if (!string.IsNullOrWhiteSpace(entry.Description))
+            {
+                <div class="desc">@entry.Description</div>
+            }
+            @if (entry.Tags.Any())
+            {
+                <div class="tags">
+                    @foreach (var tag in entry.Tags)
+                    {
+                        <span class="tag">@tag</span>
+                    }
+                </div>
+            }
+            <div class="duration">Dauer: @entry.Duration.TotalMilliseconds.ToString("0")ms</div>
+        </div>
+    }
+</div>

src/presentation/DigitalData.MessagingService.API/Pages/HealthDashboard.cshtml.cs

@@ -0,0 +1,34 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+
+namespace DigitalData.MessagingService.API.Pages;
+
+public class HealthDashboardModel(HealthCheckService healthCheckService) : PageModel
+{
+    public HealthStatus OverallStatus { get; private set; }
+    public DateTime CheckedAt { get; private set; }
+    public IReadOnlyList<HealthEntry> Entries { get; private set; } = [];
+
+    public async Task OnGetAsync()
+    {
+        var report = await healthCheckService.CheckHealthAsync();
+        CheckedAt = DateTime.UtcNow;
+        OverallStatus = report.Status;
+        Entries = report.Entries
+            .Select(e => new HealthEntry(
+                Name:        e.Key,
+                Status:      e.Value.Status,
+                Description: e.Value.Description,
+                Duration:    e.Value.Duration,
+                Tags:        e.Value.Tags))
+            .OrderBy(e => e.Status)
+            .ToList();
+    }
+
+    public record HealthEntry(
+        string Name,
+        HealthStatus Status,
+        string? Description,
+        TimeSpan Duration,
+        IEnumerable<string> Tags);
+}

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>

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

@@ -0,0 +1,3 @@
+@using DigitalData.MessagingService.API.Pages
+@namespace DigitalData.MessagingService.API.Pages
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

src/presentation/DigitalData.MessagingService.API/Program.cs

@@ -1,6 +1,10 @@
 using DigitalData.MessagingService.API.Middleware;
 using DigitalData.MessagingService.Application;
 using DigitalData.MessagingService.Infrastructure;
+using DigitalData.MessagingService.Infrastructure.Services.Background;
+using HealthChecks.UI.Client;
+using Microsoft.AspNetCore.Diagnostics.HealthChecks;
+using Microsoft.Extensions.Diagnostics.HealthChecks;
 using Serilog;
 using Serilog.Ui.Core.Extensions;
 using Serilog.Ui.SqliteDataProvider.Extensions;
@@ -66,7 +70,14 @@ try
     // Register Infrastructure layer (RabbitMQ, Repositories, etc.)
     builder.Services.AddInfrastructure(builder.Configuration);
 
+    // Health checks — each check corresponds to a singleton registered in the Infrastructure layer
+    builder.Services.AddHealthChecks()
+        .AddCheck<ConsumerInitHealthCheck>("rabbitmq-consumers",    tags: ["rabbitmq", "ready"])
+        .AddCheck<RabbitMqConnectionHealthCheck>("rabbitmq-connection", tags: ["rabbitmq", "live"])
+        .AddCheck<EmailSyncWorkerHealthCheck>("email-sync-worker",   tags: ["workers", "live"]);
+
     builder.Services.AddControllers();
+    builder.Services.AddRazorPages();
 
     builder.Services.AddEndpointsApiExplorer();
     builder.Services.AddSwaggerGen(options =>
@@ -95,6 +106,7 @@ try
                 ## Weiterführende Links
 
                 - [Serilog Log-Viewer &nearr;](/serilog-ui)
+                - [Health Status &nearr;](/health-ui)
                 """
         });
     });
@@ -155,6 +167,22 @@ try
 
     app.MapControllers();
 
+    // Health check endpoint — raw JSON, used by HealthChecks UI and monitoring tools
+    // Returns HTTP 503 if any check tagged 'ready' is Unhealthy
+    app.MapHealthChecks("/health", new HealthCheckOptions
+    {
+        Predicate      = _ => true,
+        ResponseWriter = UIResponseWriter.WriteHealthCheckUIResponse,
+        ResultStatusCodes =
+        {
+            [HealthStatus.Healthy]   = StatusCodes.Status200OK,
+            [HealthStatus.Degraded]  = StatusCodes.Status200OK,
+            [HealthStatus.Unhealthy] = StatusCodes.Status503ServiceUnavailable,
+        }
+    });
+
+    app.MapRazorPages();
+
     Log.Information("MessagingService API started successfully");
 
     app.Run();