Reorganize backend to mirror Musify.slnx folder structure

Moves the six backend projects on disk into Core/ and Hosts/
subfolders, matching the solution folders already defined in
Musify.slnx, so the file explorer reflects the same layout as the
solution:

  backend/Core/Musify.Domain
  backend/Core/Musify.Application
  backend/Core/Musify.Infrastructure
  backend/Hosts/Musify.Api
  backend/Hosts/Musify.Worker
  backend/Hosts/Musify.StreamingGateway

Updates every reference to the old paths: ProjectReference entries in
Musify.Api.csproj and Musify.Worker.csproj, the three Dockerfiles,
deploy/scripts/migrate.ps1|sh, deploy/zitadel/provision.ps1,
deploy/up.ps1, deploy/README.md and docs/development.md. Also fixes a
stale Dockerfile comment pointing at a compose file that no longer
exists.

Verified with `dotnet build Musify.slnx` against the new layout.

backend/Core/Musify.Infrastructure/Configuration/AudioConfiguration.cs

@@ -0,0 +1,41 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class AudioConfiguration
+    {
+        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/Core/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/Core/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/Core/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/Core/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/Core/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/Core/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();
+    }
+}

backend/Core/Musify.Infrastructure/Configuration/YouTubeConfiguration.cs

@@ -0,0 +1,30 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class YouTubeConfiguration
+    {
+        public const string SectionName = "YouTube";
+
+        [Required]
+        public string GeographicalLocation { get; set; } = "US";
+
+        [Range(60, 21600)]
+        public int StreamUrlCacheSeconds { get; set; } = 1800;
+
+        [Range(60, 3600)]
+        public int SearchContinuationCacheSeconds { get; set; } = 600;
+
+        [Range(0, 86400)]
+        public int SearchResultsCacheSeconds { get; set; } = 900;
+
+        [Range(0, 604800)]
+        public int SongInfoCacheSeconds { get; set; } = 86400;
+
+        [Range(32, 720)]
+        public int ThumbnailSize { get; set; } = 240;
+
+        [Range(120, 1080)]
+        public int ArtworkSize { get; set; } = 544;
+    }
+}

backend/Core/Musify.Infrastructure/Configuration/YtDlpConfiguration.cs

@@ -0,0 +1,20 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Infrastructure.Configuration
+{
+    public class YtDlpConfiguration
+    {
+        public const string SectionName = "YtDlp";
+
+        [Required]
+        public string ExecutableName { get; set; } = "yt-dlp";
+
+        [Required]
+        public string Format { get; set; } = "bestaudio";
+
+        public string AdditionalArguments { get; set; } = string.Empty;
+
+        [Required]
+        public TimeSpan DownloadTimeout { get; set; } = TimeSpan.FromMinutes(5);
+    }
+}