feat(gallery): sort/group/jump by COALESCE(post.post_date, image_record.created_at) — surface migrated content at its original publish date, not FC scan date

Operator hit this 2026-05-25 after the IR tag_apply landed: ~57k images
all scanned into FC in the same week share image_record.created_at, so
the gallery timeline collapses them into a single month bucket and
scroll orders them all together at the top. Their actual publish dates
(spread over years) were already available in Post.post_date but the
gallery never read it.

Backend wire-up:
- tag_apply phase 4 now sets ImageRecord.primary_post_id when creating
  ImageProvenance (only if currently NULL — preserves the canonical
  download-time linkage set by the importer for new FC ingests).
- gallery_service.py introduces _effective_date_col() =
  COALESCE(post.post_date, image_record.created_at), used in:
    * scroll() ORDER BY + cursor WHERE clauses
    * timeline() year/month group-by
    * jump_cursor() year/month filter
    * _neighbors() prev/next ordering
- Each method LEFT OUTER JOIN Post on primary_post_id so the COALESCE
  works for images without a post (NULL on the Post side, fall back
  to created_at).
- GalleryImage gains posted_at + effective_date fields; API /gallery
  /scroll exposes both alongside the existing created_at so the UI
  can render 'Posted on X (imported Y)' if desired.
- get_image_with_tags() returns posted_at for the modal.

Cursor format unchanged — the encoded datetime is now the effective_
date (whichever column won the COALESCE) and pagination remains
consistent.

To pick up new behavior for an already-migrated IR set: re-run
/api/migrate/tag_apply on the existing manifest (phase 4 is
idempotent; the new primary_post_id assignment backfills).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/migrators/tag_apply.py

@@ -122,7 +122,14 @@ async def _ensure_provenance(
     db: AsyncSession, *,
     image_id: int, post_id: int, source_id: int, dry_run: bool,
 ) -> bool:
-    """Returns True if a new ImageProvenance row was inserted."""
+    """Returns True if a new ImageProvenance row was inserted.
+
+    Also sets ImageRecord.primary_post_id to this post if the image
+    doesn't already have one — preserves any primary_post_id already
+    assigned at download time by the importer (don't clobber). This is
+    the linkage gallery_service.py uses to surface Post.post_date as
+    the image's effective date for sort/group/jump/neighbor nav.
+    """
     existing = (await db.execute(
         select(ImageProvenance.id).where(
             ImageProvenance.image_record_id == image_id,
@@ -130,6 +137,18 @@ async def _ensure_provenance(
             ImageProvenance.source_id == source_id,
         )
     )).scalar_one_or_none()
+
+    # Whether-or-not the provenance row already exists, ensure the
+    # image's primary_post_id is set so the gallery date-coalesce works.
+    # Idempotent: only writes when currently NULL.
+    if not dry_run:
+        await db.execute(
+            ImageRecord.__table__.update()
+            .where(ImageRecord.id == image_id)
+            .where(ImageRecord.primary_post_id.is_(None))
+            .values(primary_post_id=post_id)
+        )
+
     if existing is not None:
         return False
     if dry_run: