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

backend/app/services/gallery_service.py

@@ -55,6 +55,25 @@ def _effective_date_col():
     return ImageRecord.effective_date
 
 
+# Sort key -> the materialized column the gallery orders + cursors on. Both are
+# indexed (DESC, id DESC), so every sort is a forward index range scan.
+#   newest/oldest    → effective_date    (primary post's date, else download)
+#   posted_new/_old  → earliest_post_date (earliest publish across ALL posts)
+_SORT_COLUMNS = {
+    "newest": ImageRecord.effective_date,
+    "oldest": ImageRecord.effective_date,
+    "posted_new": ImageRecord.earliest_post_date,
+    "posted_old": ImageRecord.earliest_post_date,
+}
+_ASCENDING_SORTS = {"oldest", "posted_old"}
+
+
+def _sort_column(sort: str):
+    """The materialized date column a gallery sort orders/cursors on (falls back
+    to effective_date for any unknown sort)."""
+    return _SORT_COLUMNS.get(sort, ImageRecord.effective_date)
+
+
 def _outer_join_primary_post(stmt: Select) -> Select:
     """LEFT JOIN Post on ImageRecord.primary_post_id so the COALESCE
     above sees Post.post_date when available. Images without a post
@@ -406,7 +425,10 @@ class GalleryService:
             tag_ids, post_id, artist_id, tag_or_groups, tag_exclude,
         )
 
-        eff = _effective_date_col()
+        # eff is the ACTIVE sort column (effective_date or earliest_post_date);
+        # the cursor, ordering and year/month grouping all key off it, so the
+        # 'post date' sort paginates + buckets by original publish transparently.
+        eff = _sort_column(sort)
         stmt = select(ImageRecord, Post.post_date, eff.label("eff"))
         stmt = _outer_join_primary_post(stmt)
         stmt = _apply_scope(
@@ -417,7 +439,7 @@ class GalleryService:
             date_from=date_from, date_to=date_to,
         )
 
-        descending = sort != "oldest"
+        descending = sort not in _ASCENDING_SORTS
         if cursor:
             cur_ts, cur_id = decode_cursor(cursor)
             # The cursor is just (last eff, last id); the request's sort