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

surfsense_backend/app/file_storage/keys.py

@@ -0,0 +1,27 @@
+"""Object-key construction for stored document files."""
+
+from __future__ import annotations
+
+import os
+import uuid
+
+from app.file_storage.persistence.enums import DocumentFileKind
+
+
+def build_document_file_key(
+    *,
+    search_space_id: int,
+    document_id: int,
+    kind: DocumentFileKind,
+    filename: str,
+) -> str:
+    """Build the storage key for one document file.
+
+    Shape: ``documents/{search_space_id}/{document_id}/{kind}/{uuid}{ext}``.
+    """
+    extension = os.path.splitext(filename)[1].lower()
+    unique = uuid.uuid4().hex
+    return (
+        f"documents/{search_space_id}/{document_id}/"
+        f"{kind.value.lower()}/{unique}{extension}"
+    )