fix: CodeReview Gemini 模型從已下架 preview 換成 gemini-2.5-flash

services/code_review_pipeline_service.py:41 OPENCLAW_MODEL 預設值
gemini-2.5-flash-preview-05-20 已被 Google 下架（404 not found），
改為穩定 GA 版本 gemini-2.5-flash。

驗證：直接 API 測試
- gemini-2.5-flash → 200 ✅
- gemini-2.5-flash-preview-05-20 → 404 ❌

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/code_review_pipeline_service.py

@@ -38,7 +38,7 @@ _current_pipeline: Dict[str, Any] = {}
 _pipeline_lock = threading.Lock()
 
 GEMINI_API_KEY = os.getenv("GEMINI_API_KEY", "")
-REVIEW_MODEL   = os.getenv("OPENCLAW_MODEL", "gemini-2.5-flash-preview-05-20")
+REVIEW_MODEL   = os.getenv("OPENCLAW_MODEL", "gemini-2.5-flash")
 INTERNAL_TOKEN = os.getenv("INTERNAL_WEBHOOK_TOKEN", "")