fix: natively downloaded images take their post's date, not their download time (#4431)

The native ingesters (Discord, Patreon, SubscribeStar) import a post's
media before its record. Only the record (`_post.json`) carries the
date: the Discord message timestamp, or the Patreon/SubscribeStar
published_at. So each image was linked to a post with no date yet, and
it kept its download time in both gallery date columns. The post itself
was dated correctly once the record landed, but nothing went back to
update its images.

- upsert_post_record now re-dates the images already linked to the
  post. `effective_date` becomes the primary post's date, and
  `earliest_post_date` the earliest dated post the image is in, which
  are the same rules _attach_provenance applies.
- Migration 0113 repairs the library from the posts. It writes only
  rows that differ.

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

tests/test_sidecar_import.py

@@ -382,3 +382,36 @@ def test_external_links_not_duplicated_on_reimport(importer, import_layout):
     assert importer.session.execute(
         select(func.count()).select_from(ExternalLink)
     ).scalar_one() == 1
+
+
+def test_post_record_redates_images_linked_before_it(importer, import_layout):
+    """#4431: the native ingesters import a message's media before its record,
+    and only the record carries the date. The images start on their download
+    time; when the record lands they take the post's date."""
+    import_root, _ = import_layout
+    artist = Artist(name="Alice", slug="alice")
+    importer.session.add(artist)
+    importer.session.flush()
+    m = import_root / "Alice" / "20240301_123_01_art.jpg"
+    _split(m, "v")
+    _sidecar(m, {"category": "discord", "message_id": "123"})
+    r = importer.import_one(m)
+    assert r.status == "imported"
+    rec = importer.session.get(ImageRecord, r.image_id)
+    post = importer.session.execute(select(Post)).scalar_one()
+    assert post.post_date is None
+    download_time = rec.effective_date
+
+    sc = import_root / "Alice" / "20240301_123_post.json"
+    sc.write_text(json.dumps({
+        "category": "discord", "message_id": "123", "message": "",
+        "date": "2024-03-01T18:30:00.000000+00:00",
+    }))
+    assert importer.upsert_post_record(sc, artist=artist) is True
+    importer.session.expire_all()
+    rec = importer.session.get(ImageRecord, r.image_id)
+    post = importer.session.execute(select(Post)).scalar_one()
+    assert post.post_date is not None
+    assert post.post_date != download_time
+    assert rec.effective_date == post.post_date
+    assert rec.earliest_post_date == post.post_date