Add custom backend configuration

docs/config.md

@@ -39,11 +39,39 @@ do not override a prior value. Raw API-key fields are not accepted.
 | `server.max_artifact_bytes` | `16777216` | Maximum HTTP file-input artifact bytes; `0` disables the limit. |
 | `server.max_response_bytes` | `16777216` | Maximum encoded HTTP response bytes; `0` disables the limit. |
 | `defaults.render_format` | `text` | Default prepared-run output format: `text` or `json`. |
+| `backends` | unset | Optional mapping of custom Promptkit backend IDs to engine-scoped connection and capacity settings. |
 
 The size fields must be zero or greater. The [HTTP API](api.md) defines how
 each limit is enforced and reported. `server.artifact_root` configures an HTTP
 deployment boundary; see [operations](operations.md) for deployment handling.
 
+## Custom Backends
+
+Use `backends` when a profile selects an application-defined backend ID:
+
+```yaml
+backends:
+  local-gpu:
+    endpoint: http://localhost:11434/v1
+    api_key_env: LOCAL_GPU_API_KEY
+    extra_params:
+      provider_option: enabled
+    concurrency_limit: 2
+    queue_capacity: 0
+```
+
+Each mapping key is the case-sensitive backend ID. `endpoint` is required;
+`api_key_env`, `extra_params`, `concurrency_limit`, and `queue_capacity` are
+optional. `concurrency_limit: 0` leaves the backend unlimited. Omitting
+`queue_capacity` lets Promptkit use its default for a limited backend, while
+an explicit `queue_capacity: 0` disables queueing.
+
+Configuration strictly owns the YAML shape and rejects unknown fields. Promptkit
+validates backend IDs, endpoints, environment-variable names, extra parameters,
+and capacity relationships when Scriptorium constructs its engine. There are no
+backend command-line overrides. Store only an environment-variable name in
+`api_key_env`; raw API-key fields are not accepted.
+
 ## Framework Source Mapping
 
 Scriptorium passes `prompt_dir`, `profile_dir`, and `schema_dir` to Promptkit