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/lock/lock.go

@@ -74,6 +74,22 @@ func (l *Lock) AcquireLLM(ctx context.Context) error {
 	return nil
 }
 
+// TryAcquireLLM acquires one in-flight LLM slot without waiting and
+// reports whether it succeeded. It fails when an image job is active or
+// pending.
+func (l *Lock) TryAcquireLLM() bool {
+	l.mu.Lock()
+	if l.imageActive || len(l.imageQ) > 0 {
+		l.mu.Unlock()
+		return false
+	}
+	l.n++
+	n := l.n
+	l.mu.Unlock()
+	l.logTransition("lock transition", "state", StateLLM, "llm_inflight", n)
+	return true
+}
+
 // ReleaseLLM marks one LLM request as finished.
 func (l *Lock) ReleaseLLM() {
 	l.mu.Lock()