feat(llm): 添加 sensenova 提供商，替换 opencode-go

_API_KEYS/_FALLBACK 添加 sensenova，DB 中设 is_active=true
与 opencode-go 同模型（deepseek-v4-flash），但走 sensenova 通道

platform/backend/app/core/nvidia_client.py

@@ -24,12 +24,14 @@ class LLMError(Exception):
 _API_KEYS = {
     "opencode-go": os.getenv("OPENCODE_API_KEY", ""),
     "nvidia": os.getenv("LLM_API_KEY", ""),
+    "sensenova": os.getenv("SENSENOVA_API_KEY", ""),
 }
 
 # 代码级回退默认值（实际配置优先从 DB 读取）
 _FALLBACK = {
     "opencode-go": {"base_url": "https://opencode.ai/zen/go/v1", "model": "deepseek-v4-flash"},
     "nvidia": {"base_url": "https://integrate.api.nvidia.com/v1", "model": "stepfun-ai/step-3.5-flash"},
+    "sensenova": {"base_url": "https://token.sensenova.cn/v1", "model": "deepseek-v4-flash"},
 }
 
 def _get_active_provider() -> str: