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

backend/app/services/importer.py

@@ -1130,9 +1130,51 @@ class Importer:
         if post.artist_id is None:
             post.artist_id = artist.id
         self._apply_post_fields(post, sd)
+        self._redate_post_images(post)
         self.session.commit()
         return True
 
+    def _redate_post_images(self, post: Post) -> None:
+        """Carry a post's date onto the images already linked to it (#4431).
+
+        The native ingesters import a post's media BEFORE its record: the
+        per-media sidecar holds only the image identity (post-first, #856), and
+        the date arrives with `_post.json`. So `_attach_provenance` links each
+        image to a post that has no date yet, and the image keeps its download
+        time. This runs when the record lands, and applies the same two rules
+        `_attach_provenance` applies: `effective_date` is the PRIMARY post's
+        date, and `earliest_post_date` is the earliest date across every post
+        the image is linked to. Only rows that differ are written."""
+        if post.post_date is None:
+            return
+        self.session.flush()
+        self.session.execute(
+            update(ImageRecord)
+            .where(ImageRecord.primary_post_id == post.id)
+            .where(ImageRecord.effective_date.is_distinct_from(post.post_date))
+            .values(effective_date=post.post_date)
+            .execution_options(synchronize_session=False)
+        )
+        linked = select(ImageProvenance.image_record_id).where(
+            ImageProvenance.post_id == post.id
+        )
+        earliest = (
+            select(func.min(Post.post_date))
+            .select_from(ImageProvenance)
+            .join(Post, Post.id == ImageProvenance.post_id)
+            .where(ImageProvenance.image_record_id == ImageRecord.id)
+            .where(Post.post_date.is_not(None))
+            .correlate(ImageRecord)
+            .scalar_subquery()
+        )
+        self.session.execute(
+            update(ImageRecord)
+            .where(ImageRecord.id.in_(linked))
+            .where(ImageRecord.earliest_post_date.is_distinct_from(earliest))
+            .values(earliest_post_date=earliest)
+            .execution_options(synchronize_session=False)
+        )
+
     def attach_in_place(
         self,
         path: Path,