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

alembic/versions/0071_image_record_earliest_post_date.py

@@ -0,0 +1,80 @@
+"""image_record.earliest_post_date: original-publish gallery sort key + index
+
+Revision ID: 0071
+Revises: 0070
+Create Date: 2026-07-01
+
+effective_date (0035) keys off the PRIMARY post — which is often the repost /
+download the file actually came from — and falls back to created_at, so the
+gallery's default order surfaces download dates rather than when content was
+first posted (operator-flagged 2026-07-01). Materialize a second sort key,
+earliest_post_date = MIN(post_date) across ALL of an image's provenance posts
+(every post it appears in), falling back to created_at only when no linked post
+carries a date. Indexed (DESC, id DESC) so the "post date" gallery sort is an
+index range scan just like effective_date.
+
+Backfill mirrors 0035: created_at baseline, then override with the MIN over
+image_provenance ⋈ post. New rows get the created_at-equivalent server default;
+services/importer.py recomputes it whenever a dated post is linked.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0071"
+down_revision: Union[str, None] = "0070"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add nullable first so the backfill can populate before NOT NULL.
+    op.add_column(
+        "image_record",
+        sa.Column("earliest_post_date", sa.DateTime(timezone=True), nullable=True),
+    )
+    # Baseline: download date. Set-based (no per-row binds) → immune to the
+    # 65535 bind-parameter ceiling regardless of library size.
+    op.execute(
+        """
+        UPDATE image_record
+        SET earliest_post_date = created_at
+        """
+    )
+    # Override with the earliest post_date across EVERY post the image appears
+    # in (image_provenance is the many-to-many edge; ignore posts with no date).
+    op.execute(
+        """
+        UPDATE image_record AS ir
+        SET earliest_post_date = sub.min_date
+        FROM (
+            SELECT ip.image_record_id AS iid, MIN(p.post_date) AS min_date
+            FROM image_provenance AS ip
+            JOIN post AS p ON p.id = ip.post_id
+            WHERE p.post_date IS NOT NULL
+            GROUP BY ip.image_record_id
+        ) AS sub
+        WHERE ir.id = sub.iid
+        """
+    )
+    op.alter_column(
+        "image_record",
+        "earliest_post_date",
+        nullable=False,
+        server_default=sa.text("now()"),
+    )
+    # DESC/DESC matches the gallery's ORDER BY earliest_post_date DESC, id DESC
+    # so the "post date" scroll is a forward index scan; raw SQL because
+    # alembic's column list doesn't express per-column DESC cleanly.
+    op.execute(
+        "CREATE INDEX ix_image_record_earliest_post_date "
+        "ON image_record (earliest_post_date DESC, id DESC)"
+    )
+
+
+def downgrade() -> None:
+    op.drop_index(
+        "ix_image_record_earliest_post_date", table_name="image_record"
+    )
+    op.drop_column("image_record", "earliest_post_date")