fix(nvidia): timeout 60s→30s，NVIDIA 第一保免費，失敗轉 Gemini

- nvidia_provider.py: NVIDIA_TIMEOUT 60→30s
- models.json: timeout_seconds 60→30s
- configmap: NEMOTRON_TIMEOUT_SECONDS 45→30s, fallback 恢復 nvidia 第一
目標: Nemo 有足夠時間回應(free)，失敗快速轉 Gemini(備援)，整體機制可運作

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

apps/api/src/services/nvidia_provider.py

@@ -117,8 +117,9 @@ NVIDIA_API_URL = "https://integrate.api.nvidia.com/v1/chat/completions"
 # 預設模型 (2026-03-31 ogt: 恢復為 nemotron-mini-4b-instruct)
 NVIDIA_DEFAULT_MODEL = "nvidia/nemotron-mini-4b-instruct"
 
-# 請求超時 (秒) - Nemotron 延遲 11-45s
-NVIDIA_TIMEOUT = 60.0
+# 請求超時 (秒) - 2026-04-01 ogt: 30s 讓 Nemo 有機會回應，失敗後轉 Gemini
+# 原 60s 太長；15s 太短 (Nemo 有時需 20-40s)；30s 是實用平衡點
+NVIDIA_TIMEOUT = 30.0
 
 # 重試次數
 MAX_RETRIES = 2