fix: add Gemini fallback to pool-b for Anthropic API unavailability

Pool B now tries Anthropic Haiku first, falls back to Gemini Flash
if Anthropic is down or out of credits.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: CreatmanCEO

backend/services/llm_router.py

@@ -39,14 +39,21 @@ def create_router() -> Router:
                 "api_key": settings.cerebras_api_key.get_secret_value(),
             },
         },
-        # Pool B — complex tasks
+        # Pool B — complex tasks (Anthropic primary, Gemini fallback)
         {
             "model_name": "pool-b",
             "litellm_params": {
                 "model": settings.model_pool_b_default,
                 "api_key": settings.anthropic_api_key.get_secret_value(),
             },
         },
+        {
+            "model_name": "pool-b",
+            "litellm_params": {
+                "model": settings.model_pool_a_primary,
+                "api_key": settings.gemini_api_key.get_secret_value(),
+            },
+        },
         {
             "model_name": "pool-b-upgrade",
             "litellm_params": {