Expose backend identity and capacity errors

docs/operations.md

@@ -94,6 +94,14 @@ protect request bodies, HTTP file artifacts, and encoded responses; configure
 them through the [configuration reference](config.md) and rely on the
 [HTTP API reference](api.md) for their response effects.
 
+Configured backend concurrency and queue capacity are enforced per constructed
+Promptkit engine. A `serve` process constructs one engine for its handler, so
+concurrent HTTP requests share that transient admission state. Scriptorium does
+not retain workflow state: capacity is neither durable nor a queue of resumable
+runs. When admission is exhausted, HTTP returns `503 capacity_exceeded` without
+retry timing; callers choose any retry policy that is safe for another model
+call.
+
 Before increasing a limit:
 
 1. measure representative input, generated-output, and optional raw-output