feat: add Gemini image fallback circuit breaker

api/.env.example

@@ -25,9 +25,13 @@ IMAGE_BASE_URL=https://ai.skg.com/ezlink/v1
 IMAGE_API_KEY=
 IMAGE_MODEL=gpt-image-2
 IMAGE_REQUEST_TIMEOUT_SECONDS=60
+IMAGE_FALLBACK_ENABLED=true
+IMAGE_FALLBACK_MODEL=gemini-3-pro-image-preview
+IMAGE_CIRCUIT_FAILURE_THRESHOLD=2
+IMAGE_CIRCUIT_COOLDOWN_SECONDS=600
 GPT_IMAGE_MODEL=gpt-image-2
 SUBJECT_ASSET_IMAGE_MODEL=gpt-image-2
-SUBJECT_ASSET_IMAGE_MODELS=gpt-image-2
+SUBJECT_ASSET_IMAGE_MODELS=gpt-image-2,gemini-3-pro-image-preview
 # 可选：本地网络需要代理访问 ai.skg.com 时配置；launchd 不一定继承 shell 代理变量。
 AI_HTTP_PROXY=
 YTDLP_COOKIES_FILE=