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.Api/Properties/launchSettings.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "http": {
+        "commandName": "Project",
+        "dotnetRunMessages": true,
+        "launchBrowser": false,
+        "applicationUrl": "http://192.168.0.120:5111",
+        "environmentVariables": {
+            "ASPNETCORE_ENVIRONMENT": "Development"
+        }
+    }
+  }
+}