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_redate_images_migration.py

@@ -0,0 +1,70 @@
+"""Migration 0113 (#4431): images linked to a post before its date arrived get
+the post's date back. Runs the migration's data step against real rows."""
+import importlib.util
+from datetime import UTC, datetime
+from pathlib import Path
+
+import pytest
+
+from backend.app.models import Artist, ImageProvenance, ImageRecord, Post
+from tests.factories import make_image as _img
+
+pytestmark = pytest.mark.integration
+
+_MIGRATION = (
+    Path(__file__).resolve().parents[1]
+    / "alembic" / "versions" / "0113_redate_native_images.py"
+)
+
+
+def _redate():
+    spec = importlib.util.spec_from_file_location("m0113", _MIGRATION)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    return mod.redate_images
+
+
+def _post(db, artist, epid, when):
+    p = Post(artist_id=artist.id, external_post_id=epid, post_date=when)
+    db.add(p)
+    db.flush()
+    return p
+
+
+def _link(db, img, post, *, primary):
+    db.add(ImageProvenance(image_record_id=img.id, post_id=post.id))
+    if primary:
+        img.primary_post_id = post.id
+    db.flush()
+
+
+def test_redate_images_from_their_posts(db_sync):
+    sent = datetime(2024, 3, 1, 18, 30, tzinfo=UTC)
+    earlier = datetime(2023, 1, 5, tzinfo=UTC)
+    artist = Artist(name="Alice", slug="alice")
+    db_sync.add(artist)
+    db_sync.flush()
+    undated = _post(db_sync, artist, "u1", None)
+    dated = _post(db_sync, artist, "d1", sent)
+    repost = _post(db_sync, artist, "r1", earlier)
+
+    stale = _img(db_sync, "a" * 64)          # primary post dated, image not
+    reposted = _img(db_sync, "b" * 64)       # also in an earlier post
+    orphan = _img(db_sync, "c" * 64)         # only an undated post
+    _link(db_sync, stale, dated, primary=True)
+    _link(db_sync, reposted, dated, primary=True)
+    _link(db_sync, reposted, repost, primary=False)
+    _link(db_sync, orphan, undated, primary=True)
+    orphan_before = orphan.effective_date
+
+    _redate()(db_sync.connection())
+    db_sync.expire_all()
+
+    stale = db_sync.get(ImageRecord, stale.id)
+    reposted = db_sync.get(ImageRecord, reposted.id)
+    orphan = db_sync.get(ImageRecord, orphan.id)
+    assert stale.effective_date == sent
+    assert stale.earliest_post_date == sent
+    assert reposted.effective_date == sent             # the primary post's date
+    assert reposted.earliest_post_date == earlier      # the earliest post's date
+    assert orphan.effective_date == orphan_before      # no date to take