fix: the external-link worker reads the staging-file check from utils, not tasks.maintenance

Importing tasks.maintenance made every fetch path reach the membership
roster through its sync_memberships task, which test_gated_reason
forbids. is_temp_staging_file and its constants now live in
utils.staging, imported by both.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/utils/staging.py

@@ -0,0 +1,20 @@
+"""Downloader staging files.
+
+Downloads and imports write into a staging name and `os.replace()` it into
+place when the write completes (importer, external_fetch, native_ingest_common,
+attachment_store), so a write killed midway leaves a staging file, never a
+truncated final file. Any other file is complete.
+"""
+from pathlib import Path
+
+TEMP_STAGING_SUFFIXES = (".part", ".partial")
+# megatools stages a Mega download as `.megatmp.<id>` in the target dir.
+TEMP_STAGING_PREFIXES = (".megatmp.",)
+
+
+def is_temp_staging_file(path: Path) -> bool:
+    """True for a downloader's staging file: one that never becomes library
+    content under that name, so a stale one is safe to delete."""
+    return path.suffix in TEMP_STAGING_SUFFIXES or path.name.startswith(
+        TEMP_STAGING_PREFIXES
+    )