fix: the media backup task has its own stall threshold, and its test seeds a post for a shared archive

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

tests/test_backup_media.py

@@ -7,7 +7,14 @@ from datetime import UTC, datetime, timedelta
 import pytest
 from sqlalchemy import select
 
-from backend.app.models import BackupBlob, BackupRun, ImageRecord, PostAttachment
+from backend.app.models import (
+    Artist,
+    BackupBlob,
+    BackupRun,
+    ImageRecord,
+    Post,
+    PostAttachment,
+)
 from backend.app.services import backup_media
 from backend.app.services.backup_s3 import ListPage, S3Error, S3Target, StoredObject
 
@@ -51,8 +58,15 @@ def test_the_work_list_is_every_file_once_that_the_bucket_lacks(db_sync, tmp_pat
     _image(db_sync, tmp_path, "a")
     _image(db_sync, tmp_path, "b")
     _attachment(db_sync, tmp_path, "c")
-    # The same archive held by a second post is one file to send.
-    db_sync.add(PostAttachment(sha256=_sha("c"), path=str(tmp_path / "c-copy.zip"),
+    # The same archive held by a post as well is one file to send.
+    artist = Artist(name="Bea", slug="bea")
+    db_sync.add(artist)
+    db_sync.flush()
+    post = Post(artist_id=artist.id, external_post_id="p1")
+    db_sync.add(post)
+    db_sync.flush()
+    db_sync.add(PostAttachment(post_id=post.id, sha256=_sha("c"),
+                               path=str(tmp_path / "c-copy.zip"),
                                original_filename="set.zip", ext=".zip", size_bytes=3))
     db_sync.add(BackupBlob(target=TID, sha256=_sha("b"), key="media/bb", size_bytes=3))
     # A row for another bucket doesn't count for this one.