chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Infrastructure/Configuration/AudioTranscoderConfiguration.cs

@@ -0,0 +1,41 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class AudioTranscoderConfiguration
+    {
+        public const string SectionName = "AudioTranscoder";
+
+        [Required]
+        public TimeSpan TranscodingTimeout { get; set; } = TimeSpan.FromMinutes(5);
+
+        [Required]
+        public FfmpegConfiguration Ffmpeg { get; set; } = new FfmpegConfiguration();
+    }
+
+    public class FfmpegConfiguration
+    {
+        [Required]
+        public string ExecutableName { get; set; } = "ffmpeg";
+
+        [Required]
+        public string OutputFileName { get; set; } = "audio.m4a";
+
+        [Required]
+        public string AudioCodec { get; set; } = "aac";
+
+        [Required]
+        public string AudioBitrate { get; set; } = "128k";
+
+        [Range(1, 8)]
+        public int AudioChannels { get; set; } = 2;
+
+        [Range(8000, 192000)]
+        public int AudioSampleRate { get; set; } = 48000;
+
+        [Required]
+        public string AudioProfile { get; set; } = "aac_low";
+
+        public string AdditionalArguments { get; set; } = string.Empty;
+    }
+}

backend/Musify.Infrastructure/Configuration/DatabaseConfiguration.cs

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

backend/Musify.Infrastructure/Configuration/InfrastructureStorageConfiguration.cs

@@ -0,0 +1,25 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.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/Musify.Infrastructure/Configuration/MassTransitConfiguration.cs

@@ -0,0 +1,18 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class MassTransitConfiguration
+    {
+        public const string SectionName = "MassTransit";
+
+        [Required]
+        public required string Address { get; set; }
+
+        [Required]
+        public required string Username { get; set; }
+
+        [Required]
+        public required string Password { get; set; }
+    }
+}

backend/Musify.Infrastructure/Configuration/OpenTelemetryConfiguration.cs

@@ -0,0 +1,13 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class OpenTelemetryConfiguration
+    {
+        public const string SectionName = "OpenTelemetry";
+
+        [Url]
+        [Required]
+        public required string OtlpEndpoint { get; set; }
+    }
+}

backend/Musify.Infrastructure/Configuration/StreamTicketConfiguration.cs

@@ -0,0 +1,21 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class StreamTicketConfiguration
+    {
+        public const string SectionName = "StreamTicket";
+
+        [Required]
+        public required string PrivateKeyPath { get; set; }
+
+        [Required]
+        public string Audience { get; set; } = "media-gateway";
+
+        [Required]
+        public string Issuer { get; set; } = "musify-webapi";
+
+        [Range(60, 86400)]
+        public int TicketTtlSeconds { get; set; } = 3600;
+    }
+}

backend/Musify.Infrastructure/Configuration/WorkerConfiguration.cs

@@ -0,0 +1,18 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class WorkerConfiguration
+    {
+        public const string SectionName = "Workers";
+
+        [Required]
+        public WorkerRoutes Routes { get; set; } = new WorkerRoutes();
+    }
+
+    public class WorkerRoutes
+    {
+        [Required]
+        public string TemporaryFilesDirectory { get; set; } = Path.GetTempPath();
+    }
+}