fix: 封面尺寸改为 2560x1440 横版（与书架卡片 16:9 比例匹配）

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

config/settings.py

@@ -199,7 +199,7 @@ 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'),
+    'IMAGE_SIZE': os.environ.get('VOLCENGINE_IMAGE_SIZE', '2560x1440'),
 }
 
 # Swagger/OpenAPI Settings