test: placement API tests must commit, not flush (4246)

Five of the new tests failed in CI: every one that created a
LibraryPlacementRun and then read it back through the client. The ones that
touched no rows passed.

A flush stays inside the test's own transaction, and the app under test runs
on a separate session and connection — so the endpoint queried a database
where the row did not exist yet and got its 404 / empty list honestly.
`_seed_runs` in test_api_system_backup already commits for this reason; the
idiom was there to copy and I did not look first.

Recorded in the helper's docstring rather than just fixed, since the next
person writing a create-then-fetch API test will reach for flush too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

tests/test_api_placement.py

@@ -35,9 +35,12 @@ def test_placement_runs_on_the_long_maintenance_lane():
 
 
 def _run(db, status="ready", moves=None, artist_id=None):
-    """Adds the row; the caller awaits the flush. `db` is the ASYNC session,
+    """Adds the row; the caller awaits the COMMIT.
-    so flushing here would leave an un-awaited coroutine and the row would
+
-    never reach the database."""
+    Commit, not flush: the app under test runs on its own session and
+    connection, so a flush that stays inside this test's transaction is
+    invisible to the endpoint — the row simply is not there yet. Same reason
+    `_seed_runs` in test_api_system_backup commits."""
     run = LibraryPlacementRun(
         status=status, artist_id=artist_id, moves=moves or [],
         planned_count=len(moves or []),
@@ -56,7 +59,7 @@ async def test_runs_list_omits_the_moves(client, db):
         planned_count=1, moved_count=1,
     )
     db.add(run)
-    await db.flush()
+    await db.commit()
 
     resp = await client.get("/api/cleanup/placement/runs")
     assert resp.status_code == 200
@@ -74,7 +77,7 @@ async def test_run_detail_carries_the_moves(client, db):
         planned_count=1,
     )
     db.add(run)
-    await db.flush()
+    await db.commit()
 
     resp = await client.get(f"/api/cleanup/placement/runs/{run.id}")
     assert resp.status_code == 200
@@ -109,7 +112,7 @@ async def test_plan_accepts_an_artist_scope(client, db, monkeypatch):
     )
     artist = Artist(name="Conto", slug="conto")
     db.add(artist)
-    await db.flush()
+    await db.commit()
 
     resp = await client.post(
         "/api/cleanup/placement/plan", json={"artist_id": artist.id},
@@ -123,7 +126,7 @@ async def test_apply_refuses_a_run_that_is_not_ready(client, db):
     """The gate is here as well as in the service — an applied run must not
     be re-applied by a stray POST."""
     run = _run(db, status="applied")
-    await db.flush()
+    await db.commit()
 
     resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/apply")
     assert resp.status_code == 400
@@ -133,7 +136,7 @@ async def test_apply_refuses_a_run_that_is_not_ready(client, db):
 @pytest.mark.asyncio
 async def test_revert_refuses_a_run_that_was_never_applied(client, db):
     run = _run(db, status="ready")
-    await db.flush()
+    await db.commit()
 
     resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/revert")
     assert resp.status_code == 400
@@ -150,7 +153,7 @@ async def test_apply_dispatches_for_a_ready_run(client, db, monkeypatch):
         lambda run_id: sent.update(run_id=run_id),
     )
     run = _run(db, status="ready")
-    await db.flush()
+    await db.commit()
 
     resp = await client.post(f"/api/cleanup/placement/runs/{run.id}/apply")
     assert resp.status_code == 202