feat: allow production provider bootstrap

backend/internal/jobs/provider.go

@@ -121,6 +121,7 @@ type ProviderJobBuilder struct {
 	ImageEngine, VideoEngine     string
 	ImageEngines                 map[string]ProviderTarget
 	VideoEngines                 map[string]ProviderTarget
+	UnavailableProviders         map[string]string
 	NewID                        func() string
 }
 
@@ -141,6 +142,9 @@ func (b ProviderJobBuilder) Build(_ context.Context, owner, client, capability,
 	if err != nil {
 		return CreateCommand{}, err
 	}
+	if message := strings.TrimSpace(b.UnavailableProviders[target.Provider]); message != "" {
+		return CreateCommand{}, &Error{Kind: ErrorUnavailable, Status: 503, Message: message}
+	}
 	if target.Provider == "" || target.Model == "" || b.NewID == nil {
 		return CreateCommand{}, errors.New("provider job builder is not configured")
 	}