feat: 故事生成时自动用豆包 Seedream 4.5 文生图模型生成封面

- llm_service: 添加 _generate_and_upload_cover，调用 doubao-seedream-4-5-251128
  生成 1920x1920 儿童绘本风格封面，下载后上传至 OSS，URL 随 done 事件返回
- SSE 新增 cover 阶段（progress=90），非致命异常不影响故事主流程
- settings: LLM_CONFIG 新增 IMAGE_MODEL_NAME / IMAGE_SIZE，可通过环境变量覆盖

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

apps/stories/services/llm_service.py

@@ -122,12 +122,28 @@ def generate_story_stream(characters, scenes, props):
 
         result = _parse_story_json(full_content)
 
+        # ── Generate cover image ──
+        yield _sse_event('stage', {
+            'stage': 'cover',
+            'progress': 90,
+            'message': '正在绘制故事封面...',
+        })
+
+        cover_url = ''
+        try:
+            cover_url = _generate_and_upload_cover(
+                result['title'], characters, scenes, props, config
+            )
+        except Exception as cover_err:
+            logger.warning(f'Cover generation failed (non-fatal): {cover_err}')
+
         yield _sse_event('done', {
             'stage': 'done',
             'progress': 100,
             'message': '大功告成！',
             'title': result['title'],
             'content': result['content'],
+            'cover_url': cover_url,
         })
 
     except Exception as e:
@@ -157,6 +173,64 @@ def _parse_story_json(text):
         }
 
 
+def _generate_and_upload_cover(title, characters, scenes, props, config):
+    """
+    使用豆包文生图模型生成故事封面，上传到 OSS 并返回 URL。
+    失败时抛出异常（由调用方捕获，不影响主流程）。
+    """
+    import uuid
+    import requests as req_lib
+    from datetime import datetime
+    from django.conf import settings
+    from volcenginesdkarkruntime import Ark
+
+    # Build a concise Chinese image prompt
+    elements = []
+    if characters:
+        elements.append('、'.join(characters))
+    if scenes:
+        elements.append('、'.join(scenes))
+    if props:
+        elements.append('、'.join(props))
+
+    element_str = '，'.join(elements) if elements else title
+    image_prompt = (
+        f"儿童绘本封面插画，{title}，{element_str}，"
+        "卡通可爱风格，色彩明亮鲜艳，温馨有趣，适合3-8岁儿童，高质量插画"
+    )
+
+    client = Ark(api_key=config['API_KEY'])
+    image_model = config.get('IMAGE_MODEL_NAME', 'doubao-seedream-4-5-251128')
+    image_size = config.get('IMAGE_SIZE', '1920x1920')
+
+    result = client.images.generate(
+        model=image_model,
+        prompt=image_prompt,
+        size=image_size,
+        response_format='url',
+        watermark=False,
+    )
+
+    image_url = result.data[0].url
+
+    # Download from temporary URL and upload to OSS
+    resp = req_lib.get(image_url, timeout=60)
+    resp.raise_for_status()
+
+    from utils.oss import get_oss_client
+    oss_client = get_oss_client()
+    key = f"stories/covers/{datetime.now().strftime('%Y%m%d')}/{uuid.uuid4().hex}.jpg"
+    oss_client.bucket.put_object(
+        key, resp.content,
+        headers={'Content-Type': 'image/jpeg'},
+    )
+
+    oss_config = settings.ALIYUN_OSS
+    if oss_config.get('CUSTOM_DOMAIN'):
+        return f"https://{oss_config['CUSTOM_DOMAIN']}/{key}"
+    return f"https://{oss_config['BUCKET_NAME']}.{oss_config['ENDPOINT']}/{key}"
+
+
 def _sse_event(event, data):
     """格式化 SSE 事件"""
     return f"event: {event}\ndata: {json.dumps(data, ensure_ascii=False)}\n\n"

config/settings.py

@@ -198,6 +198,8 @@ ALIYUN_PHONE_AUTH = {
 LLM_CONFIG = {
     'API_KEY': os.environ.get('VOLCENGINE_API_KEY', ''),
     'MODEL_NAME': os.environ.get('VOLCENGINE_MODEL_NAME', 'doubao-seed-1-6-lite-251015'),
+    'IMAGE_MODEL_NAME': os.environ.get('VOLCENGINE_IMAGE_MODEL_NAME', 'doubao-seedream-4-5-251128'),
+    'IMAGE_SIZE': os.environ.get('VOLCENGINE_IMAGE_SIZE', '1920x1920'),
 }
 
 # Swagger/OpenAPI Settings