fix(code-review): keep Gemini fallback opt-in

apps/api/src/core/config.py

@@ -362,7 +362,7 @@ class Settings(BaseSettings):
             raise ValueError(
                 f"OLLAMA URL host 不允許的外部域名：{host!r}（完整 URL：{v!r}）"
                 "，必須使用私網 IP 或已知 K8s Service hostname"
-            )
+            ) from None
         if not (ip.is_private or ip.is_loopback):
             raise ValueError(
                 f"OLLAMA URL 必須是私網/loopback IP、已知 K8s SVC 或 GCP 白名單 IP，"
@@ -496,6 +496,14 @@ class Settings(BaseSettings):
     )
     GEMINI_API_KEY: str = Field(default="", description="Google Gemini API key")
     CLAUDE_API_KEY: str = Field(default="", description="Anthropic Claude API key")
+    LOCAL_CODE_REVIEW_ALLOW_GEMINI_FALLBACK: bool = Field(
+        default=False,
+        description=(
+            "Allow LocalCodeReviewService to fall back to Gemini when the "
+            "GCP-B/Ollama code-review lane fails. Default false to avoid "
+            "unexpected cloud spend from Gitea push/PR alerts."
+        ),
+    )
     # 2026-03-29 ogt: ADR-036 Nemotron Tool Calling 整合
     NVIDIA_API_KEY: str = Field(
         default="",