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

frontend/src/components/gallery/GalleryFilterBar.vue

@@ -142,9 +142,13 @@ const tagStore = useTagStore()
 const api = useApi()
 const router = useRouter()
 
+// posted_* sort by earliest publish across ALL of an image's posts (original
+// post date); newest/oldest sort by the primary post's date, else download date.
 const SORTS = [
-  { title: 'Newest first', value: 'newest' },
-  { title: 'Oldest first', value: 'oldest' },
+  { title: 'Newest post date', value: 'posted_new' },
+  { title: 'Oldest post date', value: 'posted_old' },
+  { title: 'Newest added', value: 'newest' },
+  { title: 'Oldest added', value: 'oldest' },
 ]
 
 const selected = ref(null)
@@ -175,7 +179,7 @@ const hasActiveFilters = computed(() =>
   store.filter.tag_or.length > 0 ||
   store.filter.artist_id != null ||
   store.filter.media_type != null ||
-  store.filter.sort !== 'newest' ||
+  store.filter.sort !== 'posted_new' ||
   store.filter.similar_to != null ||
   hasRefineFilters.value
 )