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/0043_post_attachment_per_post_unique.py

@@ -1,62 +0,0 @@
-"""post_attachment: per-post sha uniqueness (empty-post flood fix)
-
-Revision ID: 0043
-Revises: 0042
-Create Date: 2026-06-08
-
-PostAttachment.sha256 was GLOBALLY unique, so a non-art file the creator attaches
-to many posts (a standard pdf/zip/link-card) only ever got ONE row — on the first
-post — leaving every later post a bare shell (no image, no attachment). The native
-Patreon backfill of Anduo surfaced 1589 such shells (operator-flagged 2026-06-08).
-
-Switch to PER-POST uniqueness: the on-disk blob stays sha-deduped, but each post
-gets its own row. Replace the unique sha256 index with a plain lookup index plus
-two partial uniques — (post_id, sha256) for real posts and (sha256) for the
-NULL-post filesystem case (still one row per file there).
-
-Existing data has ≤1 row per sha (the old global unique), so the new partial
-uniques can't be violated on upgrade — no data backfill needed here. The bare-post
-shells themselves are removed by the separate prune-empty-posts cleanup tool.
-"""
-from typing import Sequence, Union
-
-import sqlalchemy as sa
-from alembic import op
-
-revision: str = "0043"
-down_revision: Union[str, None] = "0042"
-branch_labels: Union[str, Sequence[str], None] = None
-depends_on: Union[str, Sequence[str], None] = None
-
-
-def upgrade() -> None:
-    # Drop the global unique index; recreate it as a plain (non-unique) lookup
-    # index so sha-based reads keep their index (matches the model's index=True).
-    op.drop_index("ix_post_attachment_sha256", table_name="post_attachment")
-    op.create_index(
-        "ix_post_attachment_sha256", "post_attachment", ["sha256"],
-    )
-    op.create_index(
-        "uq_post_attachment_post_sha", "post_attachment",
-        ["post_id", "sha256"], unique=True,
-        postgresql_where=sa.text("post_id IS NOT NULL"),
-    )
-    op.create_index(
-        "uq_post_attachment_null_post_sha", "post_attachment",
-        ["sha256"], unique=True,
-        postgresql_where=sa.text("post_id IS NULL"),
-    )
-
-
-def downgrade() -> None:
-    op.drop_index(
-        "uq_post_attachment_null_post_sha", table_name="post_attachment"
-    )
-    op.drop_index(
-        "uq_post_attachment_post_sha", table_name="post_attachment"
-    )
-    op.drop_index("ix_post_attachment_sha256", table_name="post_attachment")
-    op.create_index(
-        "ix_post_attachment_sha256", "post_attachment", ["sha256"],
-        unique=True,
-    )