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>

backend/app/models/image_record.py

@@ -74,6 +74,17 @@ class ImageRecord(Base):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
+    # Denormalized gallery sort key = COALESCE(primary post's post_date,
+    # created_at) (alembic 0035). The gallery used to compute this as a
+    # COALESCE across the Post outer join on every /scroll, which can't use
+    # an index and re-sorted a large slice of the library per page (×10 with
+    # the old serial batching). Materializing it lets the cursor scroll read
+    # ix_image_record_effective_date directly. Maintained by the importer
+    # (services/importer.py _apply_sidecar) when a primary post with a date
+    # is linked; plain inserts keep the created_at-equivalent server default.
+    effective_date: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         nullable=False,