fix(cfai): handle Llama 4 Scout direct JSON array response in Generate()

Llama 4 Scout returns the model output directly as a JSON array in the
'response' field rather than as a plain string or [{"generated_text":"..."}].
The old fallback parsed it as [{"generated_text":""}] (Go JSON ignores
unknown fields) and silently returned an empty string — causing chapter-names
jobs to finish with results:null in the payload despite items_done matching
items_total.

Fix: only accept the generated_text fallback when the value is non-empty;
otherwise return the raw JSON bytes as a string so callers (parseChapterTitlesJSON
etc.) can extract what they need.

backend/internal/cfai/text.go

@@ -237,14 +237,18 @@ func (c *textGenHTTPClient) Generate(ctx context.Context, req TextRequest) (stri
 	if err := json.Unmarshal(wrapper.Result.Response, &text); err == nil {
 		return text, nil
 	}
-	// Fall back: array of objects with a "generated_text" field.
+	// Fall back: array of objects with a "generated_text" field
+	// (older CF AI models return [{"generated_text":"..."}]).
 	var arr []struct {
 		GeneratedText string `json:"generated_text"`
 	}
-	if err := json.Unmarshal(wrapper.Result.Response, &arr); err == nil && len(arr) > 0 {
+	if err := json.Unmarshal(wrapper.Result.Response, &arr); err == nil && len(arr) > 0 && arr[0].GeneratedText != "" {
 		return arr[0].GeneratedText, nil
 	}
-	return "", fmt.Errorf("cfai/text: model %s: unrecognised response shape: %s", req.Model, wrapper.Result.Response)
+	// Final fallback: model returned the result directly as a JSON value
+	// (e.g. Llama 4 Scout returns [{"number":1,"title":"..."},...] directly).
+	// Return the raw JSON bytes as a string so callers can parse it themselves.
+	return string(wrapper.Result.Response), nil
 }
 
 // Models returns all supported text generation model metadata.