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.

internal/config/config.go

@@ -37,6 +37,13 @@ type Config struct {
 	UpdateRepo     string
 	UpdateAsset    string
 
+	// LLMBusyMode is "wait" (hold requests until the lock is free or
+	// LLMWaitTimeout expires) or "reject" (immediately answer with
+	// LLMBusyStatus + Retry-After when an image job is active or pending).
+	LLMBusyMode    string
+	LLMBusyStatus  int
+	BusyRetryAfter int
+
 	WarmModel string
 	LogLevel  slog.Level
 	LogJSON   bool
@@ -70,6 +77,10 @@ func Defaults() Config {
 		UpdateRepo:     "https://git.rambossek.at/PUBLIC/gpu-turnstile",
 		UpdateAsset:    "gpu-turnstile.exe",
 
+		LLMBusyMode:    "wait",
+		LLMBusyStatus:  503,
+		BusyRetryAfter: 30,
+
 		LogLevel: slog.LevelWarn,
 	}
 }
@@ -169,6 +180,26 @@ func Load(getenv func(string) string) (Config, error) {
 		}
 		cfg.AutoUpdate = b
 	}
+	if v := getenv("LLM_BUSY_MODE"); v != "" {
+		if v != "wait" && v != "reject" {
+			return cfg, fmt.Errorf("LLM_BUSY_MODE: must be \"wait\" or \"reject\"")
+		}
+		cfg.LLMBusyMode = v
+	}
+	if v := getenv("LLM_BUSY_STATUS"); v != "" {
+		n, err := strconv.Atoi(v)
+		if err != nil || n < 400 || n > 599 {
+			return cfg, fmt.Errorf("LLM_BUSY_STATUS: must be an HTTP status in 400-599")
+		}
+		cfg.LLMBusyStatus = n
+	}
+	if v := getenv("BUSY_RETRY_AFTER"); v != "" {
+		n, err := strconv.Atoi(v)
+		if err != nil || n <= 0 {
+			return cfg, fmt.Errorf("BUSY_RETRY_AFTER: must be a positive integer (seconds)")
+		}
+		cfg.BusyRetryAfter = n
+	}
 	// LOGLEVEL is the canonical spelling; LOG_LEVEL is kept as an alias.
 	logLevelValue := getenv("LOGLEVEL")
 	if logLevelValue == "" {