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/Services/UploadIntentValidator.cs

@@ -0,0 +1,63 @@
+using Ardalis.Result;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Configuration;
+using Musify.Domain.Entities;
+using Musify.Domain.ValueObjects;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Services;
+
+public sealed class UploadIntentValidator(
+    IDatabase database,
+    IStorageService storageService)
+{
+    public async Task<Result> CheckQuotaAsync(
+        UploadIntentConfiguration config,
+        long userId,
+        long requiredBytes,
+        int requiredIntentCount,
+        CancellationToken cancellationToken)
+    {
+        var activeIntents = await database.UploadIntents
+            .AsNoTracking()
+            .Where(i => i.UserId == userId && i.Status == UploadIntentStatus.Issued)
+            .ToListAsync(cancellationToken);
+
+        if (activeIntents.Count + requiredIntentCount > config.MaxActiveUploadIntentsPerUser)
+            return Result.Invalid(new ValidationError("Upload intent limit exceeded. Wait for existing uploads to complete or expire."));
+
+        var activeBytes = activeIntents.Sum(i => i.ExpectedSizeBytes ?? config.DefaultExpectedPictureSizeBytes);
+        if (activeBytes + requiredBytes > config.MaxActiveUploadBytesPerUser)
+            return Result.Invalid(new ValidationError("Upload byte quota exceeded. Wait for existing uploads to complete or expire."));
+
+        return Result.Success();
+    }
+
+    public async Task<Result<UploadIntent>> ValidateAndLoadAsync(
+        UploadIntentConfiguration config,
+        Guid intentId,
+        long userId,
+        CancellationToken cancellationToken)
+    {
+        var intent = await database.UploadIntents
+            .FirstOrDefaultAsync(i => i.Id == intentId, cancellationToken);
+
+        if (intent is null || intent.UserId != userId)
+            return Result.NotFound("Upload intent not found or not accessible.");
+
+        if (intent.Status == UploadIntentStatus.Consumed)
+            return Result.Conflict("Upload intent has already been consumed.");
+
+        if (intent.Status == UploadIntentStatus.Expired || intent.ExpiresAt < DateTime.UtcNow)
+            return Result.Invalid(new ValidationError("Upload intent has expired."));
+
+        var metadata = await storageService.HeadObjectAsync(intent.Bucket, intent.Key, cancellationToken);
+        if (metadata is null)
+            return Result.NotFound("Uploaded object not found in storage. Upload the file first.");
+
+        if (metadata.ContentLength > config.MaxUploadBytes)
+            return Result.Invalid(new ValidationError($"Uploaded file exceeds the maximum allowed size of {config.MaxUploadBytes} bytes."));
+
+        return Result.Success(intent);
+    }
+}