Several keys for one outside provider

An external model takes api_key_files, each a file and an optional weight,
in place of api_key_file. Each key is an endpoint of its own
(external:MODEL#1, #2), so the router's scoring, retries and breakers work
per key. A key answering 429 is passed over while its breaker is open. A
key answering 401 is passed over too, and external_key_refused is sent,
naming the endpoint and never the key; with a single key the 401 is still
the client's. A key whose file cannot be read is left out, and usage
records which key answered by its endpoint.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/reference.md

@@ -105,10 +105,13 @@ rather than repeated.
 | `url` | `string` | [External](#external) |
 | `model` | `string` | [External](#external) |
 | `api_key_file` | `string` | [External](#external) |
+| `api_key_files` | `[]ExternalKey` | [External](#external) |
 | `peer` | `string` | [External](#external) |
 | `api` | `string` | [External](#external) |
 | `workflow` | `string` | [External](#external) |
 | `output` | `string` | [External](#external) |
+| `file` | `string` | [ExternalKey](#externalkey) |
+| `weight` | `float64` | [ExternalKey](#externalkey) |
 | `budgets` | `[]FacilityBudget` | [Facility](#facility) |
 | `margin` | `float64` | [Facility](#facility) |
 | `gpu_watts` | `float64` | [Facility](#facility) |
@@ -1103,6 +1106,15 @@ APIKeyFile holds the provider's key, on the control plane. A file
 rather than the key itself, so the key is in neither the catalog, the
 store nor the audit log; empty for a provider that asks for none.
 
+### `api_key_files`
+
+`[]ExternalKey`
+
+APIKeyFiles are several keys for the provider, in place of
+APIKeyFile: one endpoint each (external:MODEL#1, #2…), so a key that
+answers 429 or 401 is passed over while its breaker is open and the
+request goes to another. A provider's limits are per key.
+
 ### `peer`
 
 `string`
@@ -1135,6 +1147,20 @@ plane.
 
 Output is what it makes: pictures, or meshes (see ComfyUI.Output).
 
+## ExternalKey
+
+### `file`
+
+`string`
+
+File holds the key, on the control plane, as APIKeyFile does.
+
+### `weight`
+
+`float64`
+
+Weight is its share of the requests against the others'; zero is 1.
+
 ## Facility
 
 ### `budgets`