db: collapse alembic 0001..0087 into one baseline (milestone 328 step 1)

87 revisions narrating this project's build-out become one file that
creates the schema in a single step. They cost nothing at runtime — all
86 upgrade steps ran in 0.2s (note #3260) — so this is a presentation
change, not a performance one: a new installer should not inherit our
development history to stand up a database.

Deleted: 87 revisions (6,052 lines), the 10 tests/test_migration_*.py
files (483 lines) that asserted intermediate states and backfills which
no longer exist, and backend/app/utils/artist_backfill.py — the only
live module a migration imported, with no other consumer anywhere. That
last one satisfies the operator's separate request to inline it into
0008 and delete the module; the squash removes both outright.

THE REVISION ID IS "0087", NOT "0001", ON PURPOSE. It is the id of the
last revision collapsed, so an existing database is already at head and
`alembic upgrade head` does nothing. The alternative is `alembic stamp`
against live data, and stamp validates NOTHING — it writes a version
string whether or not the schema matches, so a wrong baseline surfaces
later, via the next real migration, with no clean way back. This removes
that operation rather than making it safe. Future revisions run from
0088.

Four things are hand-written because SQLAlchemy metadata does not carry
them, and none fail at generation time:

  1. CREATE EXTENSION vector          — the VECTOR columns cannot be
     created without it, so it is ordered first in upgrade().
  2. CREATE EXTENSION tsm_system_rows — surfaces only when the random
     sample query runs.
  3. the HNSW index on image_record.siglip_embedding, raw SQL because
     create_index cannot express USING hnsw (... vector_cosine_ops).
     The quietest of the four: everything works, similarity search just
     stops using an index.
  4. import pgvector.sqlalchemy.vector — autogenerate EMITS
     pgvector.sqlalchemy.vector.VECTOR references without importing it,
     so the generated file dies with NameError on first run.

The candidate came out of CI (run 4967) as checksummed base64 rather
than a plain cat, because run 4964's cat was truncated mid-line inside a
column definition with the step still green — 29 tables instead of 42,
and it looked entirely plausible. Verified here: 56,582 bytes,
sha256 471acfca69c0…, 42 tables, 66 indexes, 42 drops.

NOT YET PROVEN against the old chain. baseline.yml does that, and it is
step 2's gate; this commit does not claim the schemas match.

alembic/versions/0047_series_chapter_dividers.py

@@ -1,175 +0,0 @@
-"""series chapters become cosmetic dividers; pages become one series-global run
-
-FC-6.x reframe (#789). A series is now ONE flat, series-global ordered run of
-pages; chapters stop owning pages and become labeled dividers anchored to the
-page that begins them.
-
-Migration (order matters — series_page.chapter_id cascades, so it must be
-dropped BEFORE any chapter row is deleted, or pages would cascade away):
-  a. Renumber series_page.page_number to a series-global 1..N (ordered by the
-     OLD (chapter_number, page_number)).
-  b. Add series_chapter.anchor_page_id and populate it with each chapter's first
-     page (lowest new page_number).
-  c. Drop series_page.chapter_id (severs the cascade link).
-  d. Prune chapters that shouldn't become dividers: empty/placeholder ones (no
-     anchor) and the redundant unlabeled chapter that would sit at page 1.
-  e. Reshape series_chapter into the divider: drop chapter_number,
-     is_placeholder, stated_page_start/end; make anchor_page_id NOT NULL +
-     UNIQUE + FK→series_page ON DELETE CASCADE.
-
-Revision ID: 0047
-Revises: 0046
-Create Date: 2026-06-11
-
-"""
-from typing import Sequence, Union
-
-import sqlalchemy as sa
-from alembic import op
-
-revision: str = "0047"
-down_revision: Union[str, None] = "0046"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def upgrade() -> None:
-    # a. series-global page numbering, preserving the old reading order.
-    op.execute(
-        """
-        WITH ordered AS (
-            SELECT sp.id,
-                   ROW_NUMBER() OVER (
-                       PARTITION BY sp.series_tag_id
-                       ORDER BY sc.chapter_number, sp.page_number, sp.id
-                   ) AS rn
-            FROM series_page sp
-            JOIN series_chapter sc ON sc.id = sp.chapter_id
-        )
-        UPDATE series_page sp
-        SET page_number = ordered.rn
-        FROM ordered
-        WHERE sp.id = ordered.id
-        """
-    )
-
-    # b. anchor each existing chapter at its first page (lowest new page_number).
-    op.add_column(
-        "series_chapter",
-        sa.Column("anchor_page_id", sa.Integer(), nullable=True),
-    )
-    op.execute(
-        """
-        WITH firsts AS (
-            SELECT DISTINCT ON (sp.chapter_id)
-                   sp.chapter_id, sp.id AS page_id
-            FROM series_page sp
-            ORDER BY sp.chapter_id, sp.page_number, sp.id
-        )
-        UPDATE series_chapter sc
-        SET anchor_page_id = firsts.page_id
-        FROM firsts
-        WHERE firsts.chapter_id = sc.id
-        """
-    )
-
-    # c. sever the ownership link (drops the FK + index with the column) BEFORE
-    #    pruning chapters, so deleting a chapter can't cascade-delete its pages.
-    op.drop_column("series_page", "chapter_id")
-
-    # d. prune chapters that don't become dividers: placeholders / empty ones
-    #    (no anchor), and the unlabeled chapter that would land redundantly at
-    #    page 1 (the series just starts — no divider needed there).
-    op.execute(
-        """
-        DELETE FROM series_chapter sc
-        USING (
-            SELECT sc2.id
-            FROM series_chapter sc2
-            LEFT JOIN series_page sp ON sp.id = sc2.anchor_page_id
-            WHERE sc2.anchor_page_id IS NULL
-               OR (sp.page_number = 1
-                   AND sc2.title IS NULL
-                   AND sc2.stated_part IS NULL)
-        ) gone
-        WHERE sc.id = gone.id
-        """
-    )
-
-    # e. reshape into the divider model.
-    op.drop_column("series_chapter", "chapter_number")
-    op.drop_column("series_chapter", "is_placeholder")
-    op.drop_column("series_chapter", "stated_page_start")
-    op.drop_column("series_chapter", "stated_page_end")
-    op.alter_column("series_chapter", "anchor_page_id", nullable=False)
-    op.create_unique_constraint(
-        "uq_series_chapter_anchor_page", "series_chapter", ["anchor_page_id"]
-    )
-    op.create_foreign_key(
-        "fk_series_chapter_anchor_page",
-        "series_chapter",
-        "series_page",
-        ["anchor_page_id"],
-        ["id"],
-        ondelete="CASCADE",
-    )
-
-
-def downgrade() -> None:
-    # Lossy: dividers can't be reconstructed as owning chapters. Collapse back to
-    # exactly one chapter per series that owns all its pages in order.
-    op.add_column(
-        "series_page", sa.Column("chapter_id", sa.Integer(), nullable=True)
-    )
-    op.drop_constraint(
-        "fk_series_chapter_anchor_page", "series_chapter", type_="foreignkey"
-    )
-    op.drop_constraint(
-        "uq_series_chapter_anchor_page", "series_chapter", type_="unique"
-    )
-    op.drop_column("series_chapter", "anchor_page_id")
-    op.add_column(
-        "series_chapter",
-        sa.Column(
-            "chapter_number", sa.Integer(), nullable=False, server_default="1"
-        ),
-    )
-    op.add_column(
-        "series_chapter",
-        sa.Column(
-            "is_placeholder", sa.Boolean(), nullable=False,
-            server_default="false",
-        ),
-    )
-    op.add_column(
-        "series_chapter",
-        sa.Column("stated_page_start", sa.Integer(), nullable=True),
-    )
-    op.add_column(
-        "series_chapter",
-        sa.Column("stated_page_end", sa.Integer(), nullable=True),
-    )
-    op.execute("DELETE FROM series_chapter")
-    op.execute(
-        """
-        INSERT INTO series_chapter (series_tag_id, chapter_number)
-        SELECT DISTINCT series_tag_id, 1 FROM series_page
-        """
-    )
-    op.execute(
-        """
-        UPDATE series_page sp
-        SET chapter_id = sc.id
-        FROM series_chapter sc
-        WHERE sc.series_tag_id = sp.series_tag_id
-        """
-    )
-    op.alter_column("series_page", "chapter_id", nullable=False)
-    op.create_foreign_key(
-        "fk_series_page_chapter",
-        "series_page",
-        "series_chapter",
-        ["chapter_id"],
-        ["id"],
-        ondelete="CASCADE",
-    )