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

alembic/versions/0113_redate_native_images.py

@@ -0,0 +1,60 @@
+"""Re-date images whose post's date arrived after they were linked.
+
+#4431. The native ingesters import a post's media before its record, and the
+date travels in the record (`_post.json`). Every natively downloaded image was
+therefore linked to an undated post and kept its download time in both gallery
+date columns, while the post itself was dated correctly. The importer now
+re-dates a post's images when its record lands; this repairs the images that
+landed before that.
+
+Both columns get back the rules the importer keeps:
+
+* `effective_date` is the primary post's date (left alone when that post has
+  none, as the importer does);
+* `earliest_post_date` is the earliest dated post the image is linked to.
+
+Only rows that differ are written. The downgrade does nothing: the old values
+were download times that no one chose.
+
+Revision ID: 0113
+Revises: 0112
+Create Date: 2026-09-25
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0113"
+down_revision = "0112"
+branch_labels = None
+depends_on = None
+
+
+def redate_images(conn) -> None:
+    """The data step, on a plain connection, so a test can run it directly."""
+    conn.execute(sa.text("""
+        UPDATE image_record ir SET effective_date = p.post_date
+        FROM post p
+        WHERE p.id = ir.primary_post_id
+          AND p.post_date IS NOT NULL
+          AND ir.effective_date IS DISTINCT FROM p.post_date
+    """))
+    conn.execute(sa.text("""
+        UPDATE image_record ir SET earliest_post_date = m.earliest
+        FROM (
+          SELECT ip.image_record_id, MIN(p.post_date) AS earliest
+          FROM image_provenance ip JOIN post p ON p.id = ip.post_id
+          WHERE p.post_date IS NOT NULL
+          GROUP BY ip.image_record_id
+        ) m
+        WHERE m.image_record_id = ir.id
+          AND ir.earliest_post_date IS DISTINCT FROM m.earliest
+    """))
+
+
+def upgrade():
+    redate_images(op.get_bind())
+
+
+def downgrade():
+    pass