refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Musify.Api/DataTransferObjects/PlayLists/CreatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record CreatePlayListRequest(
+    string Name,
+    string Description,
+    Guid? PictureIntentId);

Musify.Api/DataTransferObjects/PlayLists/RequestPlayListPictureUploadRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record RequestPlayListPictureUploadRequest(
+    string FileType,
+    string ContentType,
+    long? ExpectedSizeBytes = null);

Musify.Api/DataTransferObjects/PlayLists/UpdatePlayListRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.PlayLists;
+
+public record UpdatePlayListRequest(
+    string? NewName,
+    string? NewDescription,
+    Guid? NewPictureIntentId);

Musify.Api/DataTransferObjects/Tracks/CreateTrackRequest.cs

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+namespace Musify.Api.DataTransferObjects.Tracks;
+
+public record CreateTrackRequest(
+    string Title,
+    Guid PictureIntentId,
+    Guid AudioIntentId);

Musify.Api/DataTransferObjects/Tracks/RequestTrackUploadUrlsRequest.cs

@@ -0,0 +1,9 @@
+namespace Musify.Api.DataTransferObjects.Tracks;
+
+public record RequestTrackUploadUrlsRequest(
+    string PictureFileType,
+    string PictureContentType,
+    string AudioFileType,
+    string AudioContentType,
+    long? ExpectedPictureSizeBytes = null,
+    long? ExpectedAudioSizeBytes = null);

Musify.Api/DataTransferObjects/Users/CreateUserRequest.cs

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+namespace Musify.Api.DataTransferObjects.Users;
+
+public record CreateUserRequest(
+    long Id,
+    string Name,
+    string? FirstName,
+    string? SecondName);