Add Ping backend

.NET 10 solution for Ping, a Discord style chat app: Domain, Application
and Infrastructure projects plus the Api host. SignalR for chat and
presence, LiveKit for call tokens, EF Core on Postgres, S3 compatible
storage for attachments, and OIDC auth shared with the Musify Zitadel
instance. Includes the Compose stack for local development.

backend/Core/Ping.Infrastructure/Configuration/DatabaseConfiguration.cs

@@ -0,0 +1,12 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Ping.Infrastructure.Configuration
+{
+    public class DatabaseConfiguration
+    {
+        public const string SectionName = "Database";
+
+        [Required]
+        public required string ConnectionString { get; set; }
+    }
+}

backend/Core/Ping.Infrastructure/Configuration/InfrastructureStorageConfiguration.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace Ping.Infrastructure.Configuration
+{
+    public class InfrastructureStorageConfiguration
+    {
+        public const string SectionName = "InfrastructureStorage";
+
+        [Url]
+        [Required]
+        public required string Address { get; set; }
+
+        [Required]
+        public required string AccessKey { get; set; }
+
+        [Required]
+        public required string SecretAccessKey { get; set; }
+
+        [Required]
+        public bool ForcePathStyle { get; set; }
+
+        [Required]
+        public bool UseHttp { get; set; }
+    }
+}

backend/Core/Ping.Infrastructure/Configuration/LiveKitConfiguration.cs

@@ -0,0 +1,17 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Ping.Infrastructure.Configuration
+{
+    public class LiveKitConfiguration
+    {
+        public const string SectionName = "LiveKit";
+
+        [Required]
+        public required string ApiKey { get; set; }
+
+        [Required]
+        public required string ApiSecret { get; set; }
+
+        public int TokenTtlMinutes { get; set; } = 240;
+    }
+}