Revert "db: collapse alembic 0001..0087 into one baseline"

This reverts 2529b51. Not a retreat — a reordering, on the operator's
call, and the better sequence.

The squash's acceptance test (run 4971) found ~130 places where the ORM
models do not describe the deployed schema (#3275), including a
unique=True the database never had and two UNIQUE indexes that exist
only in migrations. Collapsing now would have baked all of that into the
one file a public installer starts from.

So: fix the drift first as ordinary migrations on the intact chain, let
the operator deploy so their database moves to the corrected head, and
only then collapse. The baseline is then generated from reconciled
models and reproduces a schema worth reproducing.

Nothing is lost by reverting. The baseline was never deployed, and
regenerating it after the fixes is strictly better than patching this
copy — it will come out of autogenerate correct rather than needing the
same hand-finishing twice.

backend/app/utils/artist_backfill.py

@@ -0,0 +1,44 @@
+"""Literal SQL for the FC-2d-vii-c artist backfill / artist-tag delete.
+
+Intentionally pure string constants — NO model/slug imports, NO logic —
+so migration 0008 and its test share one drift-proof source of truth.
+Backfill steps are ordered primary -> provenance -> artist-tag and each
+only touches rows still NULL (idempotent, first match wins). The
+artist-tag step matches Artist.name = Tag.name: the importer always
+created both from the same artist_name string.
+"""
+
+BACKFILL_PRIMARY_SQL = """
+UPDATE image_record AS ir
+SET artist_id = s.artist_id
+FROM post p
+JOIN source s ON s.id = p.source_id
+WHERE ir.primary_post_id = p.id
+  AND ir.artist_id IS NULL
+"""
+
+BACKFILL_PROVENANCE_SQL = """
+UPDATE image_record AS ir
+SET artist_id = s.artist_id
+FROM (
+    SELECT DISTINCT ON (ip.image_record_id)
+           ip.image_record_id, src.artist_id
+    FROM image_provenance ip
+    JOIN source src ON src.id = ip.source_id
+    ORDER BY ip.image_record_id, ip.id
+) AS s
+WHERE ir.id = s.image_record_id
+  AND ir.artist_id IS NULL
+"""
+
+BACKFILL_TAG_SQL = """
+UPDATE image_record AS ir
+SET artist_id = a.id
+FROM image_tag it
+JOIN tag t ON t.id = it.tag_id AND t.kind = 'artist'
+JOIN artist a ON a.name = t.name
+WHERE it.image_record_id = ir.id
+  AND ir.artist_id IS NULL
+"""
+
+DELETE_ARTIST_TAGS_SQL = "DELETE FROM tag WHERE kind = 'artist'"