Expose backend identity and capacity errors

docs/api.md

@@ -100,13 +100,13 @@ validation contract. The response contains:
   optional `uri`;
 - `validation`: `status`, `mode`, `repair_attempts`, `is_valid`, plus
   optional `errors` and `schema_path`;
-- `metadata`: run, prompt, rendered-prompt, profile, model, input-hash, usage,
+- `metadata`: run, prompt, rendered-prompt, profile, optional backend identity, model, input-hash, usage,
   timing, validation, and repair-attempt metadata; and
 - optional `raw_model_output` when requested.
 
 `metadata.model_params` has `endpoint`, `model`, `temperature`,
 `max_tokens`, `top_p`, and `timeout_seconds`, plus optional
-`service_tier`, `reasoning_effort`, `api_key_env`, and `extra_params`.
+`backend_id`, `service_tier`, `reasoning_effort`, `api_key_env`, and `extra_params`.
 `metadata.usage` always includes `prompt_tokens`, `completion_tokens`,
 `total_tokens`, `cached_tokens`, and `cache_write_tokens`; unavailable
 cache usage is reported as zero.
@@ -115,6 +115,11 @@ When Promptkit resolves a direct or definition-rendered session ID,
 `metadata.session_id` contains that effective result value. It is omitted when
 no effective session ID exists.
 
+`metadata.selected_backend_id` and `metadata.model_params.backend_id` report
+the corresponding Promptkit result fields independently when present. Both are
+omitted for an endpoint-only profile; Scriptorium does not infer backend
+identity from an endpoint.
+
 A validation failure has `validation.status: "failed"`, `is_valid: false`,
 and any available diagnostic errors, while still returning the artifact and
 metadata.
@@ -150,6 +155,7 @@ Messages are concise and do not expose wrapped internal causes.
 | `500` | `validation_runtime_failed` | Schema or validator runtime failure. |
 | `500` | `internal_error` | Unclassified server failure. |
 | `502` | `llm_failed` | Outbound model request failed. |
+| `503` | `capacity_exceeded` | The selected model backend has no admission capacity. No retry timing is supplied. |
 
 ## Retry And Idempotency