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.Application/Tracks/Handlers/GetTrackStreamQueryHandler.cs

@@ -0,0 +1,54 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Configuration;
+using Musify.Application.Contracts;
+using Musify.Application.Tracks.Queries;
+using Musify.Application.Tracks.Responses;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Tracks.Handlers
+{
+    public class GetTrackStreamQueryHandler(
+        IDatabase database,
+        IStreamTicketService ticketService,
+        TrackConfiguration trackConfiguration,
+        StreamGatewayConfiguration streamGatewayConfiguration,
+        ILogger<GetTrackStreamQueryHandler> logger)
+        : IQueryHandler<GetTrackStreamQuery, Result<TrackStreamResponse>>
+    {
+        public async ValueTask<Result<TrackStreamResponse>> Handle(GetTrackStreamQuery request, CancellationToken cancellationToken)
+        {
+            var track = await database.Tracks.AsNoTracking()
+                .Where(t => t.Id == request.TrackId)
+                .Select(t => new { t.Id, t.AudioTranscodeProcessingStatus, t.AudioFolderName })
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (track is null)
+                return Result<TrackStreamResponse>.NotFound();
+
+            if (string.IsNullOrWhiteSpace(track.AudioFolderName)
+                || track.AudioTranscodeProcessingStatus != ProcessingStatus.Completed)
+            {
+                logger.LogInformation("Stream requested for track {TrackId} but audio is not ready", request.TrackId);
+                return Result<TrackStreamResponse>.Conflict("Track audio is not available for streaming yet.");
+            }
+
+            var folderPath = trackConfiguration.Routes.BuildProcessedAudioPath(track.AudioFolderName);
+            var keyPrefix = $"{folderPath}/";
+
+            var ticket = ticketService.IssueTicket(request.UserId, keyPrefix);
+
+            var manifestUrl = string.Join('/',
+                streamGatewayConfiguration.PublicBaseUrl.TrimEnd('/'),
+                "media",
+                folderPath,
+                streamGatewayConfiguration.AudioFileName);
+
+            logger.LogInformation("Issued stream ticket for track {TrackId} to user {UserId}", request.TrackId, request.UserId);
+
+            return Result.Success(new TrackStreamResponse(manifestUrl, ticket.Token, ticket.ExpiresInSeconds));
+        }
+    }
+}