feat(ai): promote Nvidia nemotron as default arbitrator for high complexity/risk incidents

apps/api/src/services/ai_router.py

@@ -180,11 +180,12 @@ class AIRouter:
         }
 
         # 完整 Fallback 鏈 (Provider, Model)
-        # 2026-03-29 ogt: NVIDIA 不在一般 Fallback 鏈 (僅用於 Tool Calling)
+        # 2026-03-30 ogt: NVIDIA 成為首選仲裁，加入 Fallback 鏈首位
         self._full_fallback_chain: list[tuple[AIProvider, str]] = [
-            (AIProvider.OLLAMA, self._ollama_default),
+            (AIProvider.NVIDIA, self._nvidia_default),
             (AIProvider.GEMINI, self._gemini_default),
             (AIProvider.CLAUDE, self._claude_default),
+            (AIProvider.OLLAMA, self._ollama_default),
         ]
 
         # Tool Calling 專用 Fallback 鏈 (ADR-036)
@@ -351,12 +352,12 @@ class AIRouter:
             return provider, model, reason
 
         # =======================================================================
-        # 規則 4: 複雜度 4-5 或 HIGH 風險 → Gemini
+        # 規則 4: 複雜度 4-5 或 HIGH 風險 → Nvidia Nemotron
         # =======================================================================
         if score >= 4 or risk == RiskLevel.HIGH:
-            provider = AIProvider.GEMINI
-            model = self._gemini_default
-            reason = f"複雜度={score}/5, 風險={risk.value} → Gemini (fallback Claude)"
+            provider = AIProvider.NVIDIA
+            model = self._nvidia_default
+            reason = f"複雜度={score}/5, 風險={risk.value} → Nvidia (fallback Gemini)"
             return provider, model, reason
 
         # =======================================================================
@@ -556,8 +557,8 @@ class AIRouter:
             {
                 "rule": 4,
                 "condition": "complexity >= 4 OR HIGH risk",
-                "provider": "gemini",
-                "reason": "高複雜度需要雲端能力",
+                "provider": "nvidia",
+                "reason": "高複雜度需要 Nvidia Nemotron 強大推理能力",
             },
             {
                 "rule": 5,