fix: a non-image file lands on the source it was downloaded for, and the undated shells it left are folded into their real posts (#4435)

The attachment path looked the post's source up by (artist, platform),
taking the artist's lowest-id source. A Discord artist has one source per
channel, so every archive or pdf from a later channel became an undated
second post on the first channel's source. _post_for_sidecar now takes
the downloading source, as upsert_post_record and _apply_sidecar already
did. Migration 0114 re-points each shell's attachments to its dated twin
and deletes the shell.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_importer_attach_in_place.py

@@ -435,3 +435,48 @@ def test_attach_in_place_non_media_routes_to_attachment(importer, db_sync):
     ).scalar_one()
     assert row.ext == ".txt"
     assert row.artist_id == artist.id
+
+
+def test_a_non_media_file_lands_on_the_source_being_downloaded(importer, db_sync):
+    """#4435: a Discord artist has one source per channel. A non-media file
+    downloaded for the SECOND channel was filed under the artist's first
+    Discord source (the (artist, platform) lookup takes the lowest id), as an
+    undated shell post; the real post record then made a second post under
+    the right source. The attachment must follow the source it was fetched for."""
+    from backend.app.models import Post, PostAttachment, Source
+
+    images_root = importer.images_root
+    artist = Artist(name="Yara", slug="yara")
+    db_sync.add(artist)
+    db_sync.flush()
+    first = Source(
+        artist_id=artist.id, platform="discord",
+        url="https://discord.com/channels/1/100",
+    )
+    second = Source(
+        artist_id=artist.id, platform="discord",
+        url="https://discord.com/channels/1/200",
+    )
+    db_sync.add_all([first, second])
+    db_sync.flush()
+
+    rar = images_root / "yara" / "discord" / "rewards" / "20241226_555_01_pack.rar"
+    rar.parent.mkdir(parents=True, exist_ok=True)
+    rar.write_bytes(b"not a real archive, so it is kept as an attachment")
+    rar.with_suffix(rar.suffix + ".json").write_text(
+        '{"category": "discord", "id": "555", "message_id": "555"}'
+    )
+
+    result = importer.attach_in_place(rar, artist=artist, source=second)
+    assert result.status == "attached"
+
+    owner = db_sync.execute(
+        select(Post.source_id, Post.external_post_id)
+        .join(PostAttachment, PostAttachment.post_id == Post.id)
+        .where(PostAttachment.original_filename == rar.name)
+    ).one()
+    assert owner.source_id == second.id
+    assert owner.external_post_id == "555"
+    assert db_sync.execute(
+        select(func.count()).select_from(Post).where(Post.source_id == first.id)
+    ).scalar_one() == 0