perf(gallery): materialize indexed effective_date sort key

The gallery cursored on COALESCE(post.post_date, image_record.created_at)
across the Post outer join — an expression spanning two tables that no
index can serve, so every /scroll sorted a large slice of the library
(and the old frontend fired ten serially). Materialize it:

- image_record.effective_date column + ix_image_record_effective_date
  (effective_date DESC, id DESC); alembic 0035 backfills
  COALESCE(primary post's post_date, created_at) for existing rows.
- gallery_service._effective_date_col() now returns the column, so scroll
  / timeline / jump / neighbors all order off the index instead of
  re-deriving the COALESCE. _neighbors reads record.effective_date
  directly (drops an extra Post lookup).
- importer._apply_sidecar maintains it: when a primary post with a date is
  linked, effective_date = post.post_date; plain inserts keep the
  created_at-equivalent server default.

Tests: sidecar import asserts effective_date == post.post_date; gallery
ordering/timeline/jump test seeds set effective_date alongside created_at.

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

tests/test_api_gallery.py

@@ -17,6 +17,7 @@ async def _seed(db, count: int = 3):
             origin="imported_filesystem", integrity_status="unknown",
         )
         r.created_at = base - timedelta(minutes=i)
+        r.effective_date = r.created_at  # no post → tracks created_at (0035)
         db.add(r)
     await db.flush()
     await db.commit()

tests/test_gallery_artist_payload.py

@@ -14,6 +14,7 @@ async def _img(db, n):
         origin="imported_filesystem", integrity_status="unknown",
     )
     rec.created_at = datetime.now(UTC) - timedelta(minutes=n)
+    rec.effective_date = rec.created_at  # no post → tracks created_at (0035)
     db.add(rec)
     await db.flush()
     return rec

tests/test_gallery_provenance_filter.py

@@ -22,6 +22,7 @@ async def _img(db, n):
         origin="imported_filesystem", integrity_status="unknown",
     )
     rec.created_at = base - timedelta(minutes=n)
+    rec.effective_date = rec.created_at  # no primary post → tracks created_at (0035)
     db.add(rec)
     await db.flush()
     return rec

tests/test_gallery_service.py

@@ -32,6 +32,8 @@ async def _seed_images(db, count: int, sha_prefix: str = "0") -> list[ImageRecor
             integrity_status="unknown",
         )
         r.created_at = base - timedelta(minutes=i)
+        # No post → effective_date == created_at (alembic 0035 denorm).
+        r.effective_date = r.created_at
         db.add(r)
         records.append(r)
     await db.flush()
@@ -166,6 +168,9 @@ async def _seed_image_with_post(
         primary_post_id=post.id,
     )
     img.created_at = image_created_at
+    # Mirror the importer's denorm (alembic 0035): a linked post with a date
+    # sets effective_date to that date, else it falls back to created_at.
+    img.effective_date = post_date or image_created_at
     db.add(img)
     await db.flush()
     return img, post
@@ -201,6 +206,7 @@ async def test_scroll_sorts_by_post_date_when_available(db):
         origin="imported_filesystem", integrity_status="unknown",
     )
     img_c.created_at = base_import - timedelta(days=5)
+    img_c.effective_date = img_c.created_at  # no post → tracks created_at
     db.add(img_c)
     await db.flush()
 

tests/test_sidecar_import.py

@@ -94,6 +94,9 @@ def test_sidecar_creates_provenance(importer, import_layout):
     prov = importer.session.execute(select(ImageProvenance)).scalar_one()
     assert prov.image_record_id == rec.id and prov.post_id == post.id
     assert rec.primary_post_id == post.id
+    # Denormalized gallery sort key (alembic 0035) tracks the primary post's
+    # publish date so /scroll orders off ix_image_record_effective_date.
+    assert rec.effective_date == post.post_date
 
 
 def test_reimport_same_post_idempotent(importer, import_layout):