Add LLM busy modes: wait (hang) or reject with Retry-After

LLM_BUSY_MODE=reject answers blocked LLM requests immediately with
LLM_BUSY_STATUS (default 503, 429 works) and Retry-After, so routers
like LiteLLM can cool down and retry instead of holding a hung
connection. The default wait mode now also sends Retry-After when
LLM_WAIT_TIMEOUT expires. Document the service account (LocalSystem
default, NT SERVICE virtual-account hardening) and the Program
Files / ProgramData install layout.

cmd/gpu-turnstile/main.go

@@ -192,6 +192,9 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 		"unload_timeout", cfg.UnloadTimeout,
 		"job_timeout", cfg.JobTimeout,
 		"llm_wait_timeout", cfg.LLMWaitTimeout,
+		"llm_busy_mode", cfg.LLMBusyMode,
+		"llm_busy_status", cfg.LLMBusyStatus,
+		"busy_retry_after", cfg.BusyRetryAfter,
 		"unload_poll_interval", cfg.UnloadPollInterval,
 		"history_poll_interval", cfg.HistoryPollInterval,
 		"probe_timeout", cfg.ProbeTimeout,
@@ -238,6 +241,9 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 		HistoryPollInterval: cfg.HistoryPollInterval,
 		FreeTimeout:         cfg.FreeTimeout,
 		WarmTimeout:         cfg.WarmTimeout,
+		LLMBusyMode:         cfg.LLMBusyMode,
+		LLMBusyStatus:       cfg.LLMBusyStatus,
+		BusyRetryAfter:      cfg.BusyRetryAfter,
 		BackoffInitial:      cfg.BackoffInitial,
 		BackoffMax:          cfg.BackoffMax,
 		PromptCaptureLimit:  cfg.PromptCaptureLimit,