fix(reconcile): preserve from_attachment_id when merging duplicate posts (#73/#87)

Milestone #73 (reconcile duplicate gallery-dl/native post rows) shipped in
eff6427; closing it out after today's #87 work, which added a seam it didn't
account for. _repoint_post_links drops a loser post's ImageProvenance row on the
(image, post) uniqueness collision — and that row may now carry from_attachment_id
(which archive the file was extracted from). For the exact gallery-dl->native
case this targets, the keeper is the native stub (no archive) and the loser is the
gallery-dl row that extracted the member, so a blind delete silently lost the
containing-archive linkage. Carry from_attachment_id onto the keeper's surviving
row (when NULL) before dropping the collision.

The rarer PostAttachment-collision case (both dup posts captured the same archive
blob) doesn't arise in the targeted scenario — the archive lives only on the
gallery-dl post, so it re-points straight to the keeper and the FK stays valid.

Test: collision merge preserves the loser's from_attachment_id on the keeper.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/cleanup_service.py

@@ -573,6 +573,29 @@ def _repoint_post_links(session: Session, loser_id: int, keeper_id: int) -> None
     dup_imgs = select(ImageProvenance.image_record_id).where(
         ImageProvenance.post_id == keeper_id
     )
+    # Before dropping the colliding loser rows, carry their from_attachment_id
+    # (which archive the file came out of, milestone #87) onto the keeper's
+    # surviving row when the keeper didn't record one. For the gallery-dl→native
+    # case this very milestone targets, the keeper is the native stub (no
+    # archive) and the loser is the gallery-dl row that extracted the member, so
+    # a blind delete would silently lose the containing-archive linkage.
+    for img_id, att_id in session.execute(
+        select(ImageProvenance.image_record_id, ImageProvenance.from_attachment_id)
+        .where(
+            ImageProvenance.post_id == loser_id,
+            ImageProvenance.image_record_id.in_(dup_imgs),
+            ImageProvenance.from_attachment_id.is_not(None),
+        )
+    ).all():
+        session.execute(
+            update(ImageProvenance)
+            .where(
+                ImageProvenance.post_id == keeper_id,
+                ImageProvenance.image_record_id == img_id,
+                ImageProvenance.from_attachment_id.is_(None),
+            )
+            .values(from_attachment_id=att_id)
+        )
     session.execute(
         delete(ImageProvenance).where(
             ImageProvenance.post_id == loser_id,