feat(ai): add OpenRouter provider order and fallback controls

app/services/ai_generation.py

@@ -325,7 +325,7 @@ async def call_openrouter_api(
         "X-Title": "IRT Bank Soal",
     }
 
-    payload = {
+    payload: dict[str, Any] = {
         "model": model,
         "messages": [
             {
@@ -336,6 +336,14 @@ async def call_openrouter_api(
         "max_tokens": 2000,
         "temperature": 0.7,
     }
+    provider_order = [
+        provider for provider in settings.OPENROUTER_PROVIDER_ORDER if provider.strip()
+    ]
+    if provider_order:
+        payload["provider"] = {
+            "order": provider_order,
+            "allow_fallbacks": settings.OPENROUTER_ALLOW_PROVIDER_FALLBACKS,
+        }
 
     timeout = httpx.Timeout(settings.OPENROUTER_TIMEOUT)