fix: handle Llama 4 Scout array response shape in CF AI text decoder

Llama 4 Scout returns `result.response` as an array of objects
[{"generated_text":"..."}] instead of a plain string. Decode into
json.RawMessage and try both shapes; fall back to generated_text[0].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/cfai/text.go

@@ -217,9 +217,11 @@ func (c *textGenHTTPClient) Generate(ctx context.Context, req TextRequest) (stri
 	}
 
 	// CF AI wraps responses: { "result": { "response": "..." }, "success": true }
+	// Some models (e.g. Llama 4 Scout) return response as an array:
+	// { "result": { "response": [{"generated_text":"..."}] } }
 	var wrapper struct {
 		Result struct {
-			Response string `json:"response"`
+			Response json.RawMessage `json:"response"`
 		} `json:"result"`
 		Success bool     `json:"success"`
 		Errors  []string `json:"errors"`
@@ -230,7 +232,19 @@ func (c *textGenHTTPClient) Generate(ctx context.Context, req TextRequest) (stri
 	if !wrapper.Success {
 		return "", fmt.Errorf("cfai/text: model %s error: %v", req.Model, wrapper.Errors)
 	}
-	return wrapper.Result.Response, nil
+	// Try plain string first.
+	var text string
+	if err := json.Unmarshal(wrapper.Result.Response, &text); err == nil {
+		return text, nil
+	}
+	// Fall back: array of objects with a "generated_text" field.
+	var arr []struct {
+		GeneratedText string `json:"generated_text"`
+	}
+	if err := json.Unmarshal(wrapper.Result.Response, &arr); err == nil && len(arr) > 0 {
+		return arr[0].GeneratedText, nil
+	}
+	return "", fmt.Errorf("cfai/text: model %s: unrecognised response shape: %s", req.Model, wrapper.Result.Response)
 }
 
 // Models returns all supported text generation model metadata.