feat(gallery): sort by earliest post date across all posts (new default)

The gallery's newest/oldest sort keys off image_record.effective_date =
COALESCE(primary post's post_date, created_at). The primary post is often the
repost/download the file came from, so the grid led with download dates rather
than when content was first posted (operator-flagged).

Add a second materialized sort key, earliest_post_date = MIN(post_date) across
ALL of an image's provenance posts (every post it appears in), else created_at —
the original publish date. Mirrors the effective_date pattern so the sort stays a
forward index scan.

- alembic 0071: add earliest_post_date + index (DESC, id DESC); backfill
  created_at baseline then MIN over image_provenance ⋈ post.
- importer: recompute earliest_post_date whenever a dated post is linked (MIN over
  the image's provenance, which now includes the just-added row).
- gallery_service: new sorts posted_new / posted_old key off earliest_post_date;
  cursor + year/month grouping follow the active column transparently.
- api: accept posted_new|posted_old; DEFAULT is now posted_new so the grid leads
  with original publish date. newest/oldest (effective_date) still available.
- frontend: sort dropdown gains "Newest/Oldest post date" (default Newest post
  date); existing effective-date sorts relabelled "Newest/Oldest added".
- tests: service test asserts posted_new/posted_old key off earliest_post_date;
  frontend default-sort omission test updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

tests/test_gallery_service.py

@@ -73,6 +73,28 @@ async def test_scroll_returns_newest_first(db):
     assert page.images[0].created_at > page.images[-1].created_at
 
 
+@pytest.mark.asyncio
+async def test_scroll_sorts_by_earliest_post_date(db):
+    """posted_new/posted_old key off earliest_post_date (original publish across
+    all of an image's posts), NOT effective_date/created_at — so an old file that
+    was first posted recently sorts by that recent post date."""
+    imgs = await _seed_images(db, 3)
+    now = _now()
+    # earliest_post_date order deliberately DIFFERS from created_at order (which
+    # _seed_images makes imgs[0] newest → imgs[2] oldest), proving the sort keys
+    # off earliest_post_date.
+    imgs[0].earliest_post_date = now - timedelta(days=1)     # middle
+    imgs[1].earliest_post_date = now - timedelta(days=365)   # oldest post
+    imgs[2].earliest_post_date = now - timedelta(hours=1)    # newest post
+    await db.flush()
+
+    svc = GalleryService(db)
+    newest = await svc.scroll(cursor=None, limit=10, sort="posted_new")
+    assert [i.id for i in newest.images] == [imgs[2].id, imgs[0].id, imgs[1].id]
+    oldest = await svc.scroll(cursor=None, limit=10, sort="posted_old")
+    assert [i.id for i in oldest.images] == [imgs[1].id, imgs[0].id, imgs[2].id]
+
+
 @pytest.mark.asyncio
 async def test_scroll_pagination(db):
     await _seed_images(db, 5)