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.

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;
+    }
+}