Add backend capacity policy configuration

internal/domain/domain.go

@@ -163,10 +163,20 @@ type PromptMessageTemplate struct {
 
 // Backend describes reusable OpenAI-compatible connection defaults.
 type Backend struct {
-	ID          string
-	Endpoint    string
-	APIKeyEnv   string
-	ExtraParams map[string]any
+	ID               string
+	Endpoint         string
+	APIKeyEnv        string
+	ExtraParams      map[string]any
+	ConcurrencyLimit int
+	QueueCapacity    int
+	QueueCapacitySet bool
+}
+
+// BackendCapacityPolicy describes normalized run and generation capacity for
+// one limited backend.
+type BackendCapacityPolicy struct {
+	ConcurrencyLimit int
+	QueueCapacity    int
 }
 
 // ExecutionProfile describes how and where to execute a model.