fix: text-gen chapter-names truncation and bad title format

- Default max_tokens to 4096 for chapter-names so large chapter lists
  are not cut off mid-JSON by the model's token limit
- Rewrite system prompt to clarify placeholder semantics ({n} = number,
  {scene} = scene hint) and explicitly forbid echoing the number inside
  the title field — prevents "Chapter 1 - 1: ..." style duplications
- UI: surface raw_response in the error area when chapters:[] is returned
  so the admin can see what the model actually produced

backend/internal/backend/handlers_textgen.go

@@ -99,12 +99,17 @@ func (s *Server) handleAdminTextGenChapterNames(w http.ResponseWriter, r *http.R
 	}
 
 	systemPrompt := `You are a chapter title editor for a web novel platform. ` +
-		`The user will provide a list of chapter numbers with their current titles, ` +
-		`and a naming pattern. Your task is to produce a renamed version of every chapter ` +
-		`following the pattern exactly. ` +
-		`Respond ONLY with a JSON array — no prose, no markdown fences, no explanation. ` +
-		`Each element must be an object: {"number": <int>, "title": <string>}. ` +
-		`Output every chapter in the input list. Do not skip any.`
+		`The user provides a list of chapter numbers with their current titles, ` +
+		`and a naming pattern template. ` +
+		`Your job: produce one new title for every chapter, following the pattern exactly. ` +
+		`Pattern placeholders: {n} = the chapter number (integer), {scene} = a very short (2–5 word) scene hint derived from the existing title. ` +
+		`RULES: ` +
+		`1. Do NOT include the chapter number inside the title text — the {n} placeholder is already in the pattern. ` +
+		`2. Do NOT include any prefix like "Chapter X -" or "Chapter X:" inside the title field itself. ` +
+		`3. The "title" field in your JSON must be the fully-rendered string (e.g. if pattern is "Chapter {n}: {scene}", output "Chapter 3: The Bet"). ` +
+		`4. Respond ONLY with a raw JSON array — no prose, no markdown fences, no explanation. ` +
+		`5. Each element: {"number": <int>, "title": <string>}. ` +
+		`6. Output every chapter in the input list, in order. Do not skip any.`
 
 	userPrompt := fmt.Sprintf(
 		"Naming pattern: %s\n\nChapters:\n%s",
@@ -117,8 +122,14 @@ func (s *Server) handleAdminTextGenChapterNames(w http.ResponseWriter, r *http.R
 		model = cfai.DefaultTextModel
 	}
 
+	// Default to 4096 tokens so large chapter lists are not truncated.
+	maxTokens := req.MaxTokens
+	if maxTokens <= 0 {
+		maxTokens = 4096
+	}
+
 	s.deps.Log.Info("admin: text-gen chapter-names requested",
-		"slug", req.Slug, "chapters", len(chapters), "model", model)
+		"slug", req.Slug, "chapters", len(chapters), "model", model, "max_tokens", maxTokens)
 
 	raw, genErr := s.deps.TextGen.Generate(r.Context(), cfai.TextRequest{
 		Model: model,
@@ -126,7 +137,7 @@ func (s *Server) handleAdminTextGenChapterNames(w http.ResponseWriter, r *http.R
 			{Role: "system", Content: systemPrompt},
 			{Role: "user", Content: userPrompt},
 		},
-		MaxTokens: req.MaxTokens,
+		MaxTokens: maxTokens,
 	})
 	if genErr != nil {
 		s.deps.Log.Error("admin: text-gen chapter-names failed", "err", genErr)