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/0045_image_prediction_table.py

@@ -1,69 +0,0 @@
-"""image_prediction table (DDL only — backfill runs as a background task)
-
-Normalizes the per-image tagger predictions out of the JSON blob into a
-queryable table (#768). This migration creates ONLY the table + indexes — it
-is pure DDL and commits instantly, so web boots immediately.
-
-The data backfill from the existing image_record.tagger_predictions JSON is
-deliberately NOT done here. Doing it inline made the whole migration one
-transaction over the ~100 GB TOAST: nothing committed until the very end, it
-was invisible/unmonitorable mid-run, and an early MATERIALIZED-CTE form spilled
-the full 100 GB to temp. Instead the backfill is the
-backend.app.tasks.admin.backfill_image_predictions_task — batched by id window,
-committed per chunk (visible progress + resumable), idempotent
-(ON CONFLICT DO NOTHING). Trigger it from Settings → Maintenance once web is up.
-
-The old image_record.tagger_predictions column is left in place (vestigial) and
-dropped in a follow-up once the backfill + code cutover are verified — dropping
-it needs an ACCESS EXCLUSIVE lock on the hot image_record table (the 0044 lock
-class), so it's deferred to a quiesced-worker window.
-
-Revision ID: 0045
-Revises: 0044
-Create Date: 2026-06-10
-
-"""
-from typing import Sequence, Union
-
-import sqlalchemy as sa
-from alembic import op
-
-revision: str = "0045"
-down_revision: Union[str, None] = "0044"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def upgrade() -> None:
-    op.create_table(
-        "image_prediction",
-        sa.Column("id", sa.Integer(), primary_key=True),
-        sa.Column(
-            "image_record_id", sa.Integer(),
-            sa.ForeignKey("image_record.id", ondelete="CASCADE"),
-            nullable=False,
-        ),
-        sa.Column("raw_name", sa.String(length=255), nullable=False),
-        sa.Column("category", sa.String(length=64), nullable=False),
-        sa.Column("score", sa.Float(), nullable=False),
-        sa.UniqueConstraint(
-            "image_record_id", "raw_name", name="image_raw_name",
-        ),
-    )
-    op.create_index(
-        "ix_image_prediction_image", "image_prediction", ["image_record_id"],
-    )
-    op.create_index(
-        "ix_image_prediction_name_score", "image_prediction",
-        ["raw_name", "score"],
-    )
-    # No data backfill here — see the module docstring. The one-time copy from
-    # image_record.tagger_predictions runs as backfill_image_predictions_task
-    # (batched, resumable, idempotent), kept out of this transaction so web boots
-    # without waiting on a ~100 GB pass.
-
-
-def downgrade() -> None:
-    op.drop_index("ix_image_prediction_name_score", "image_prediction")
-    op.drop_index("ix_image_prediction_image", "image_prediction")
-    op.drop_table("image_prediction")