fix(ai): models.json gemini-1.5-flash → gemini-2.0-flash (404 修復)

gemini-1.5-flash 已停用，改用 gemini-2.0-flash。
models.json 上次未跟著 model_registry.py 同步更新。

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

apps/api/models.json

@@ -45,9 +45,9 @@
       "endpoint": "https://generativelanguage.googleapis.com/v1beta",
       "api_path": "/models/{model}:generateContent",
       "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"
       },
       "options": {
         "temperature": 0.1,