feat(products): SD-0002 §6.3 schema — migration 0002 + nh3/multipart deps (SLICE-5)

Also updates test_migrate_from_empty_applies_0001 → test_migrate_from_empty_applies_all
to assert both migrations apply on a fresh DB (mechanical consequence of adding 0002).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/requirements.txt

@@ -5,3 +5,5 @@ psycopg[binary]>=3.1
 psycopg-pool>=3.2
 pytest>=8.0
 import-linter>=2.0
+nh3>=0.2
+python-multipart>=0.0.9