refactor: separate provider capability policy

backend/app/services/provider_cache.py

@@ -1,22 +1,20 @@
 """Redis-backed cache for providers loaded from DB."""
 
-import json
-from collections import defaultdict
-from typing import Literal
-
-from pydantic import BaseModel
+import json
+from collections import defaultdict
+
+from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.core.logging import get_logger
-from app.core.redis import get_redis
-from app.db.admin_models import Provider
-
-logger = get_logger(__name__)
-
-ProviderType = Literal["text", "image", "tts", "storybook"]
-
-
+from app.core.redis import get_redis
+from app.db.admin_models import Provider
+from app.services.provider_policy import ProviderType
+
+logger = get_logger(__name__)
+
+
 class CachedProvider(BaseModel):
     """Serializable provider configuration matching DB model fields."""