feat/ memoir-cover (#16)

Co-authored-by: Kevin <kevin@brighteng.org>

api/app/features/memoir/memoir_images/prompting.py

@@ -93,6 +93,87 @@ class MemoirImagePromptService:
             "prompt_context": prompt_context,
         }
 
+    def build_cover_prompt(
+        self,
+        chapter_title: str,
+        chapter_category: str,
+        context_excerpt: str,
+    ) -> dict[str, str]:
+        """生成章节封面图的 image-generation prompt。"""
+        style = self.CATEGORY_STYLE_MAP.get(chapter_category, self.settings.default_style)
+        prompt_context = f"{chapter_category}: {chapter_title}"
+
+        llm_input = {
+            "chapter_title": chapter_title,
+            "chapter_category": chapter_category,
+            "context_excerpt": context_excerpt,
+            "default_style": style,
+            "default_size": self.settings.default_size,
+        }
+
+        if self.llm:
+            try:
+                response = self.llm.invoke(
+                    "Return JSON only with keys prompt, style, size. "
+                    "Create an image-generation prompt for a memoir chapter COVER. "
+                    "Emphasize: hero composition, evocative scene, chapter cover aesthetic.\n"
+                    + json.dumps(llm_input, ensure_ascii=False)
+                )
+                parsed = json.loads(extract_json_payload(response.content))
+                return {
+                    "prompt": _ensure_style_in_prompt(
+                        parsed["prompt"], parsed.get("style", style)
+                    ),
+                    "style": parsed.get("style", style),
+                    "size": parsed.get("size", self.settings.default_size),
+                    "prompt_context": prompt_context,
+                }
+            except Exception as exc:
+                logger.warning(
+                    "封面 prompt 生成回退到默认模板: chapter_category=%s, title=%s, error=%s",
+                    chapter_category,
+                    chapter_title,
+                    exc,
+                )
+
+        return {
+            "prompt": _ensure_style_in_prompt(
+                self._build_cover_fallback_prompt(
+                    chapter_category=chapter_category,
+                    context_excerpt=context_excerpt,
+                    style=style,
+                ),
+                style,
+            ),
+            "style": style,
+            "size": self.settings.default_size,
+            "prompt_context": prompt_context,
+        }
+
+    def _build_cover_fallback_prompt(
+        self,
+        chapter_category: str,
+        context_excerpt: str,
+        style: str,
+    ) -> str:
+        subject = self.CATEGORY_FALLBACK_SUBJECT_MAP.get(
+            chapter_category, "memoir scene"
+        )
+        if _contains_cjk(context_excerpt):
+            return (
+                f"A {style} chapter cover illustration of a {subject}, "
+                "hero composition, evocative scene, emotionally resonant, "
+                "cinematic framing, natural lighting, no text overlay."
+            )
+        details = (context_excerpt or "").strip()[:200]
+        if not details:
+            details = "A personal life story scene with authentic emotional detail"
+        return (
+            f"A {style} chapter cover illustration of a {subject}. "
+            f"Scene hint: {details}. "
+            "Hero composition, evocative scene, cinematic framing, no text overlay."
+        )
+
     def _build_fallback_prompt(
         self,
         chapter_category: str,