feat(file-storage): add settings, key builder, and backend factory

surfsense_backend/app/file_storage/factory.py

@@ -0,0 +1,38 @@
+"""Resolve the configured :class:`StorageBackend` as a process-wide singleton."""
+
+from __future__ import annotations
+
+from functools import lru_cache
+
+from app.file_storage.backends.base import StorageBackend
+from app.file_storage.settings import (
+    AZURE_BACKEND,
+    LOCAL_BACKEND,
+    load_storage_settings,
+)
+
+
+@lru_cache(maxsize=1)
+def get_storage_backend() -> StorageBackend:
+    """Build the backend selected by ``FILE_STORAGE_BACKEND`` (lazy-imported)."""
+    settings = load_storage_settings()
+
+    if settings.backend == AZURE_BACKEND:
+        if not settings.azure_connection_string or not settings.azure_container:
+            raise ValueError(
+                "Azure storage requires AZURE_STORAGE_CONNECTION_STRING and "
+                "AZURE_STORAGE_CONTAINER."
+            )
+        from app.file_storage.backends.azure import AzureBlobBackend
+
+        return AzureBlobBackend(
+            connection_string=settings.azure_connection_string,
+            container=settings.azure_container,
+        )
+
+    if settings.backend == LOCAL_BACKEND:
+        from app.file_storage.backends.local import LocalFileBackend
+
+        return LocalFileBackend(settings.local_root)
+
+    raise ValueError(f"Unknown FILE_STORAGE_BACKEND: {settings.backend!r}")