fix(tests): provenance itest user fixture is get-or-create — the lane DB persists across tests

Both integration tests built the same username; the second insert died
on users_username_key. 19 passed, 1 error on run 3802.

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

tests/test_snippet_provenance.py

@@ -88,10 +88,21 @@ async def _dispose_engine():
 
 @pytest_asyncio.fixture
 async def user_id(_dispose_engine):
+    # Get-or-create: the lane's database persists across tests, so a second
+    # test re-creating the same username dies on the unique constraint.
+    from sqlalchemy import select
+
     from scribe.models import async_session
     from scribe.models.user import User
 
     async with async_session() as s:
+        existing = (
+            await s.execute(
+                select(User).where(User.username == "snippet_prov_itest")
+            )
+        ).scalar_one_or_none()
+        if existing is not None:
+            return existing.id
         user = User(username="snippet_prov_itest")
         s.add(user)
         await s.flush()