fix(ai): 更新 Gemini 模型名稱 1.5-flash → 2.0-flash (2026-03-28 ogt)

根本原因: gemini-1.5-flash 已停用，API 返回 404
解決方案: 更新到 gemini-2.0-flash

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/services/model_registry.py

@@ -124,11 +124,12 @@ class ModelRegistry:
                         "summary": "llama3.2:3b",
                     }
                 },
+                # 2026-03-28 ogt: 更新到 gemini-2.0-flash (1.5 已停用)
                 "gemini": {
                     "models": {
-                        "default": "gemini-1.5-flash",
-                        "rca": "gemini-1.5-flash",
-                        "summary": "gemini-1.5-flash",
+                        "default": "gemini-2.0-flash",
+                        "rca": "gemini-2.0-flash",
+                        "summary": "gemini-2.0-flash",
                     }
                 },
                 "claude": {
@@ -176,9 +177,10 @@ class ModelRegistry:
 
         if not model:
             # 最終 fallback
+            # 2026-03-28 ogt: 更新 gemini-2.0-flash
             fallback_map = {
                 "ollama": "qwen2.5:7b-instruct",
-                "gemini": "gemini-1.5-flash",
+                "gemini": "gemini-2.0-flash",
                 "claude": "claude-3-haiku-20240307",
             }
             model = fallback_map.get(provider, provider)