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/Hosts/Musify.Api/AppSettings.Development.json

@@ -0,0 +1,47 @@
+{
+    "Logging": {
+        "LogLevel": {
+            "Default": "Information",
+            "Microsoft.AspNetCore": "Warning"
+        }
+    },
+    "Authentication": {
+        "Scopes": [ "openid", "profile", "email" ],
+        "RequireHttpsMetadata": false
+    },
+    "Database": {
+        "ConnectionString": "Host=localhost;Port=59000;Database=musify_db;Username=postgres;Password=postgres"
+    },
+    "MassTransit": {
+        "Address": "rabbitmq://localhost:5672",
+        "Username": "guest",
+        "Password": "guest"
+    },
+    "InfrastructureStorage": {
+        "Address": "http://localhost:8333",
+        "AccessKey": "admin_access_key",
+        "SecretAccessKey": "admin_secret_key",
+        "ForcePathStyle": true,
+        "UseHttp": true
+    },
+    "ApplicationStorage": {
+        "Bucket": "webapi-storage"
+    },
+    "StreamGateway": {
+        "PublicBaseUrl": "http://localhost:8081"
+    },
+    "StreamTicket": {
+        "PrivateKeyPath": "../../deploy/keys/stream_private.pem"
+    },
+    "OpenTelemetry": {
+        "OtlpEndpoint": "http://localhost:4317"
+    },
+    "YouTube": {
+        "GeographicalLocation": "ES"
+    },
+    "Workers": {
+        "Routes": {
+            "TemporaryFilesDirectory": "../../deploy/.tmp"
+        }
+    }
+}