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/0070_gpu_job_lease_indexes.py

@@ -1,44 +0,0 @@
-"""partial indexes so GPU-job leasing stays O(batch), not O(completed)
-
-The lease claims the lowest-id pending (or expired-leased) jobs. With only a
-plain `status` index, `... ORDER BY id LIMIT n` walked the primary-key index from
-the start, skipping the entire prefix of already-done/error rows before reaching
-pending ones — so leasing slowed to a crawl as `done` piled up (the whole reason
-throughput fell off a cliff mid-run and /status stalled). Two partial indexes fix
-it: the pending one is id-ordered so the hot path reads just the first n entries,
-and the leased-expiry one keeps the crash-recovery reclaim + the orphan sweep
-cheap. They cover only the small live slice of the table, so they stay tiny even
-as the done/error history grows to millions.
-
-Revision ID: 0070
-Revises: 0069
-Create Date: 2026-06-30
-"""
-from typing import Sequence, Union
-
-import sqlalchemy as sa
-from alembic import op
-
-revision: str = "0070"
-down_revision: Union[str, None] = "0069"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def upgrade() -> None:
-    # Hot path: lowest-id pending jobs. Index on id, restricted to pending, so
-    # `WHERE status='pending' ORDER BY id LIMIT n` is a short index-order scan.
-    op.create_index(
-        "ix_gpu_job_pending", "gpu_job", ["id"],
-        postgresql_where=sa.text("status = 'pending'"),
-    )
-    # Crash-recovery: expired leases, for the lease backstop + recover_orphaned.
-    op.create_index(
-        "ix_gpu_job_leased_expires", "gpu_job", ["lease_expires_at"],
-        postgresql_where=sa.text("status = 'leased'"),
-    )
-
-
-def downgrade() -> None:
-    op.drop_index("ix_gpu_job_leased_expires", table_name="gpu_job")
-    op.drop_index("ix_gpu_job_pending", table_name="gpu_job")