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()