T-529 default to cmhub gateway

app/appconfig.py

@@ -31,7 +31,7 @@ DEFAULT_CONFIG = {
         "default_text_model": "GPT-5.5 文本",
         "default_image_model": "Nano Banana 2",
         "generate_cover": False,
-        "backend": "direct",
+        "backend": "cmhub",
         "cmhub": {
             "base_url": "",
             "title_alias": "",
@@ -321,7 +321,8 @@ def ai_config(config=None) -> dict:
 
 def ai_backend(config=None) -> str:
     ai = ai_config(config)
-    backend = str(ai.get("backend", "direct") or "direct").strip().lower()
+    default_backend = DEFAULT_CONFIG["ai"]["backend"]
+    backend = str(ai.get("backend", default_backend) or default_backend).strip().lower()
     if backend not in AI_BACKENDS:
         raise ConfigError("AI backend 必须是 direct 或 cmhub")
     return backend