test: import the placement task module so its names reach celery.tasks (4246)

Three registration assertions failed: a task name only enters `celery.tasks`
when its module is imported, and nothing in the test process imported
`library_placement`. The API routes import it lazily inside the handlers, and
the registration test runs before any handler test triggers that.

`include=[...]` is what gets the module imported in a real WORKER, so
production registration was never in question — the test was asserting
something only observable after an import it never performed.

test_tasks_admin already carries the convention verbatim
(`import backend.app.tasks.admin  # noqa: F401 — register tasks`); I wrote the
assertion from what I meant instead of copying the idiom next to it. Same
mistake shape as the commit-vs-flush bounce one commit ago: the pattern was
already in the suite both times.

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

tests/test_api_placement.py

@@ -8,6 +8,7 @@ endpoints gate on run state, and that a list response stays small.
 import pytest
 from sqlalchemy import select
 
+import backend.app.tasks.library_placement  # noqa: F401 — register tasks
 from backend.app.celery_app import celery
 from backend.app.models import Artist, LibraryPlacementRun