fix: 减少图片生成数量（当前3个section对应一张图）

api/tasks/memoir_tasks.py

@@ -302,22 +302,26 @@ def _save_narrative_to_sections(db: Session, chapter, narrative: str, title: str
 
     settings = MemoirImageSettings.from_env()
     prompt_service = MemoirImagePromptService(llm=None, settings=settings) if settings.enabled else None
-    effective_max = settings.effective_max_images(len(narrative_to_parse)) if settings.enabled else 0
-    all_placeholders = [s["placeholder_info"] for s in segments if s.get("placeholder_info")]
-    placeholders = _select_placeholders_for_effective_max(
-        placeholders=all_placeholders,
-        existing_images=[],
-        effective_max=effective_max,
-    ) if settings.enabled else []
-    selected_placeholder_set = {p.get("placeholder") for p in placeholders}
 
-    # 按顺序创建 section，保证每个 section 的 content 与 image 一一对应（order_index 严格递增）
+    # 每 3 个 section 对应 1 张图片，其他 section 的 image_id 为空
+    def _should_have_image(order_idx: int) -> bool:
+        return (order_idx % 3) == 2
+
+    def _placeholder_for_segment(seg: dict, order_idx: int) -> dict | None:
+        ph = seg.get("placeholder_info")
+        if ph and ph.get("placeholder") and ph.get("description"):
+            return ph
+        content = (seg.get("content") or "").strip()
+        desc = (content[:50] + "…") if len(content) > 50 else (content or "章节配图")
+        return {"placeholder": f"{{{{{{{{IMAGE:{desc}}}}}}}}}", "description": desc}
+
+    # 按顺序创建 section，每 3 个 section 对应 1 张配图
     for i, seg in enumerate(segments):
         order_idx = order_base + i
         content = (seg.get("content") or "").strip()
-        ph = seg.get("placeholder_info")
         image_asset = None
-        if ph and settings.enabled and ph.get("placeholder") in selected_placeholder_set:
+        if settings.enabled and _should_have_image(order_idx):
+            ph = _placeholder_for_segment(seg, order_idx)
             style = prompt_service.CATEGORY_STYLE_MAP.get(category, settings.default_style) if prompt_service else settings.default_style
             image_asset = build_initial_image_assets(
                 [ph],