feat(deploy): consolidated dev stack with auto-provisioned Zitadel

Single docker-compose stack for local development (Postgres, Zitadel,
RabbitMQ, SeaweedFS + bucket init, Jaeger) plus an optional overlay to run
the .NET apps in Docker. up.ps1 orchestrates the full bootstrap: stack up,
stream-ticket keys, EF Core migrations and Zitadel provisioning.

- Zitadel is provisioned automatically (service-account PAT -> project +
  OIDC app with JWT access token); the generated ClientId is wired into
  Musify.Api user-secrets and .env. No manual console steps.
- ZITADEL_EXTERNAL_DOMAIN=host.docker.internal so token validation works
  both with local apps and full-docker.
- App Dockerfiles copy AppSettings*.json to lowercase so config loads on
  case-sensitive Linux; Worker image ships ffmpeg.
- Scripts split by responsibility (common/stream-keys/migrate/provision);
  apps overlay de-duped via YAML anchors.
- Remove the superseded standalone seaweedfs compose; point docs/development
  at the unified stack.

backend/Musify.StreamingGateway.Dockerfile

@@ -0,0 +1,17 @@
+# Build context: backend/
+FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
+WORKDIR /src
+
+COPY Directory.Build.props Directory.Packages.props nuget.config ./
+COPY Musify.StreamingGateway/Musify.StreamingGateway.csproj Musify.StreamingGateway/
+RUN dotnet restore Musify.StreamingGateway/Musify.StreamingGateway.csproj
+
+COPY . .
+RUN dotnet publish Musify.StreamingGateway/Musify.StreamingGateway.csproj -c Release -o /app --no-restore
+
+FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
+WORKDIR /app
+COPY --from=build /app ./
+ENV ASPNETCORE_URLS=http://+:8081
+EXPOSE 8081
+ENTRYPOINT ["dotnet", "Musify.StreamingGateway.dll"]