feat(health): add custom DB, Qdrant, and Neo4j health check services and secure Qdrant in staging

src/NexusReader.Web/Services/QdrantHealthCheck.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Diagnostics.HealthChecks;
+using Qdrant.Client;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace NexusReader.Web.Services;
+
+public class QdrantHealthCheck : IHealthCheck
+{
+    private readonly QdrantClient _qdrantClient;
+
+    public QdrantHealthCheck(QdrantClient qdrantClient)
+    {
+        _qdrantClient = qdrantClient;
+    }
+
+    public async Task<HealthCheckResult> CheckHealthAsync(
+        HealthCheckContext context, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            // Simple check: query collection existence to verify connection is alive
+            _ = await _qdrantClient.CollectionExistsAsync("knowledge_units", cancellationToken);
+            return HealthCheckResult.Healthy("Qdrant database is accessible.");
+        }
+        catch (Exception ex)
+        {
+            return HealthCheckResult.Unhealthy("Qdrant database health check failed.", ex);
+        }
+    }
+}