feat(etl-cache): resolve cache blob storage backend

surfsense_backend/app/etl_pipeline/cache/storage/backend.py

@@ -0,0 +1,46 @@
+"""Resolve the storage backend for cache blobs: shared main store or a dedicated one."""
+
+from __future__ import annotations
+
+from functools import lru_cache
+
+from app.file_storage.backends.base import StorageBackend
+
+
+@lru_cache(maxsize=1)
+def resolve_cache_backend() -> StorageBackend:
+    from app.etl_pipeline.cache.settings import load_etl_cache_settings
+
+    settings = load_etl_cache_settings()
+
+    if not settings.storage_backend:
+        from app.file_storage.factory import get_storage_backend
+
+        return get_storage_backend()
+
+    backend = settings.storage_backend.strip().lower()
+
+    if backend == "azure":
+        from app.config import config
+
+        if not settings.storage_container:
+            raise ValueError("ETL_CACHE_STORAGE_CONTAINER is required for azure cache.")
+        if not config.AZURE_STORAGE_CONNECTION_STRING:
+            raise ValueError(
+                "AZURE_STORAGE_CONNECTION_STRING is required for azure cache."
+            )
+        from app.file_storage.backends.azure import AzureBlobBackend
+
+        return AzureBlobBackend(
+            connection_string=config.AZURE_STORAGE_CONNECTION_STRING,
+            container=settings.storage_container,
+        )
+
+    if backend == "local":
+        if not settings.storage_local_root:
+            raise ValueError("ETL_CACHE_STORAGE_LOCAL_PATH is required for local cache.")
+        from app.file_storage.backends.local import LocalFileBackend
+
+        return LocalFileBackend(settings.storage_local_root)
+
+    raise ValueError(f"Unknown ETL_CACHE_STORAGE_BACKEND: {settings.storage_backend!r}")