fix(failover): Gemini 永遠附在 Ollama fallback 鏈尾（ADR-110 漏加）

GCP-A HEALTHY → fallback=[GCP-B, Local, Gemini]
GCP-B HEALTHY → fallback=[Local, Gemini]
與舊 111 HEALTHY → fallback=[Gemini] 行為一致，保留雲端最後防線。

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

apps/api/src/services/ollama_failover_manager.py

@@ -316,22 +316,22 @@ class OllamaFailoverManager:
 
         now_ts = datetime.datetime.now(TAIPEI_TZ).isoformat()
 
-        # GCP-A 健康 → 主 GCP-A
+        # GCP-A 健康 → 主 GCP-A，Gemini 永遠在 Ollama 鏈最後（與舊 111 行為一致）
         if health_gcp_a.status == HealthStatus.HEALTHY:
             return OllamaRoutingResult(
                 primary=ep_gcp_a,
-                fallback_chain=[ep_gcp_b, ep_local],
+                fallback_chain=[ep_gcp_b, ep_local, _GEMINI_ENDPOINT],
                 routing_reason="GCP-A HEALTHY → primary GCP-A",
                 health_gcp_a=health_gcp_a,
                 health_gcp_b=health_gcp_b,
                 health_local=health_local,
             )
 
-        # GCP-A 不健康，GCP-B 健康 → 切 GCP-B
+        # GCP-A 不健康，GCP-B 健康 → 切 GCP-B，Gemini 在鏈尾
         if health_gcp_b.status == HealthStatus.HEALTHY:
             return OllamaRoutingResult(
                 primary=ep_gcp_b,
-                fallback_chain=[ep_local],
+                fallback_chain=[ep_local, _GEMINI_ENDPOINT],
                 routing_reason=f"GCP-A {health_gcp_a.status.value} → 切 GCP-B at {now_ts}",
                 health_gcp_a=health_gcp_a,
                 health_gcp_b=health_gcp_b,