refactor(db): shim document_files.search_space_id to physical workspace_id

Phase 1 (rename DB) commit 1b: same attribute->physical column shim as
1a, applied to the document_files satellite model. ORM attribute name
unchanged; FK target string left for the searchspaces rename commit.

surfsense_backend/app/file_storage/persistence/models.py

@@ -30,6 +30,7 @@ class DocumentFile(BaseModel, TimestampMixin):
         index=True,
     )
     search_space_id = Column(
+        "workspace_id",
         Integer,
         ForeignKey("searchspaces.id", ondelete="CASCADE"),
         nullable=False,