032: fix run_translation route handler — sync def → async def (asyncio.create_task needs running loop)

run_translation was def (sync FastAPI handler runs in thread pool with no event
loop), but its body calls asyncio.create_task(_background_execute()) which
requires a running event loop. Changed to async def so FastAPI runs it on the
event loop directly.

Error: 'Run failed: no running event loop'

backend/src/api/routes/translate/_run_routes.py

@@ -27,7 +27,7 @@ from ._router import router
 # @PRE  User has translate.job.execute permission.
 # @POST  Returns the created translation run.
 @router.post("/jobs/{job_id}/run", status_code=status.HTTP_201_CREATED)
-def run_translation(
+async def run_translation(
     job_id: str,
     full_translation: bool = Query(False, description="Translate ALL rows (skip new-key-only filter)"),
     current_user: User = Depends(get_current_user),