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,

backend/app/services/gallery_service.py

@@ -43,16 +43,17 @@ def decode_cursor(cursor: str) -> tuple[datetime, int]:
 
 
 def _effective_date_col():
-    """SQL expression: COALESCE(post.post_date, image_record.created_at).
+    """The materialized gallery sort key: image_record.effective_date
+    (alembic 0035) = COALESCE(primary post's post_date, created_at),
+    maintained at write time by the importer.
 
-    Used as the canonical sort/group/filter key across the gallery so
-    images backfilled with primary_post_id (e.g. via tag_apply phase 4)
-    surface at their original publish date, not their FC import date.
-    Images without a Post (or with Post.post_date NULL) fall back to
-    image_record.created_at and still order coherently against
-    post-attached ones.
+    Canonical sort/group/filter key across the gallery so images attached
+    to a post surface at their original publish date, not their FC import
+    date — and, now that it's a single indexed column rather than a
+    COALESCE across the Post outer join, the cursor scroll is an index
+    range scan instead of a full re-sort per page.
     """
-    return func.coalesce(Post.post_date, ImageRecord.created_at)
+    return ImageRecord.effective_date
 
 
 def _outer_join_primary_post(stmt: Select) -> Select:
@@ -357,17 +358,10 @@ class GalleryService:
         }
 
     async def _neighbors(self, record: ImageRecord) -> dict:
-        # Compute the boundary image's effective_date in Python (one query
-        # below + the SELECT we already have on `record`) and use it for
-        # the neighbor comparison. Cheaper than re-deriving in SQL via
-        # correlated subquery.
-        boundary_eff = record.created_at
-        if record.primary_post_id is not None:
-            post_date = (await self.session.execute(
-                select(Post.post_date).where(Post.id == record.primary_post_id)
-            )).scalar_one_or_none()
-            if post_date is not None:
-                boundary_eff = post_date
+        # The boundary image's sort key is materialized on the row now
+        # (alembic 0035) — read it directly instead of re-deriving COALESCE
+        # via an extra Post lookup.
+        boundary_eff = record.effective_date
 
         eff = _effective_date_col()
         prev_stmt = _outer_join_primary_post(

backend/app/services/importer.py

@@ -960,6 +960,14 @@ class Importer:
                 sp.rollback()
         if record.primary_post_id is None:
             record.primary_post_id = post.id
+        # Keep the denormalized gallery sort key (alembic 0035) aligned with
+        # the primary post's publish date so /scroll orders off
+        # ix_image_record_effective_date instead of COALESCE-ing across the
+        # post join. Only override when THIS post is the primary AND carries
+        # a date; otherwise the column keeps its created_at-equivalent server
+        # default (matches the old COALESCE(post_date, created_at) fallback).
+        if record.primary_post_id == post.id and post.post_date is not None:
+            record.effective_date = post.post_date
         self.session.flush()
 
     def _copy_to_library(