feat(provenance): image_record.artist_id + 0008 backfill/drop-artist-tags

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/utils/artist_backfill.py

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+"""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'"