Fix shutdown issue and logging in HTTPServer

[pior]

• Configurable shutdown timeout — HTTPServer now returns *httpServer (still satisfies Runnable) with a fluent API: .Name(string) and .ShutdownTimeout(time.Duration). The shutdown timeout was previously hardcoded at 30s.
• Fix unnecessary Shutdown call — When ListenAndServe fails on its own (e.g. port in use), the code no longer calls Shutdown on an already-dead server.
• Clearer log messages — Graceful shutdown now logs "shutting down" → "stopped" (matching the manager's pattern). Server errors log "stopped with error" instead of the ambiguous "shutdown".
• Real unit tests — Added tests for graceful shutdown, listen error, pre-cancelled context, and fluent API configuration. Kept existing example tests.
• Updated README example logs to match actual output.