debug: add temporary /ai/provider-debug endpoint

Shows which provider is selected and whether keys are loaded.
Remove after debugging the 502 on PR env.

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

backend/app/api/endpoints/ai_builder.py

@@ -55,6 +55,33 @@ def _require_ai_enabled() -> None:
         )
 
 
+@router.get("/provider-debug")
+async def provider_debug(
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    _: None = Depends(require_engineer_or_admin),
+):
+    """Temporary debug endpoint — shows which AI provider would be selected."""
+    from app.core.ai_provider import get_ai_provider
+    has_gemini_key = bool(settings.GOOGLE_AI_API_KEY)
+    has_anthropic_key = bool(settings.ANTHROPIC_API_KEY)
+    provider_setting = settings.AI_PROVIDER
+    try:
+        provider = get_ai_provider()
+        provider_type = type(provider).__name__
+    except RuntimeError as e:
+        provider_type = f"ERROR: {e}"
+    return {
+        "ai_provider_setting": provider_setting,
+        "has_gemini_key": has_gemini_key,
+        "gemini_key_prefix": settings.GOOGLE_AI_API_KEY[:8] + "..." if settings.GOOGLE_AI_API_KEY else None,
+        "has_anthropic_key": has_anthropic_key,
+        "anthropic_key_prefix": settings.ANTHROPIC_API_KEY[:8] + "..." if settings.ANTHROPIC_API_KEY else None,
+        "selected_provider": provider_type,
+        "gemini_model": settings.AI_MODEL_GEMINI,
+        "anthropic_model": settings.AI_MODEL_ANTHROPIC,
+    }
+
+
 @router.get("/quota", response_model=AIQuotaStatusResponse)
 async def get_quota(
     current_user: Annotated[User, Depends(get_current_active_user)],