Review

Live freight checkpoint feed uses offset as cursor

Yard ops asked for infinite scroll on the live checkpoint feed so dispatchers can page through car arrivals without full reloads. This PR renames page/limit to cursor/limit and returns next_cursor so the React board can keep scrolling as railcars ping RFID gates.

FastAPITier 4paginationkeysetconcurrencyfastapi

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apps/yard_api/routers/checkpoints.py+13-10

48	48		@router.get("/yards/{yard_id}/checkpoints")
49		-	async def list_checkpoints(
50		-	yard_id: UUID,
51		-	page: int = Query(1, ge=1),
52		-	limit: int = Query(50, ge=1, le=200),
53		-	db: AsyncSession = Depends(get_db),
54		-	) -> CheckpointPage:
	49	+	async def list_checkpoints(
	50	+	yard_id: UUID,
	51	+	cursor: str \| None = Query(None, description="Opaque pagination cursor"),
	52	+	limit: int = Query(50, ge=1, le=200),
	53	+	db: AsyncSession = Depends(get_db),
	54	+	) -> CheckpointPage:
55	55		await require_yard_access(db, yard_id)
56		-	offset = (page - 1) * limit
	56	+	# cursor is the page index clients already fetched; default to first page
	57	+	page = int(cursor) if cursor is not None else 1
	58	+	offset = (page - 1) * limit
57	59		stmt = (
58	60		select(CheckpointEvent)
59	61		.where(CheckpointEvent.yard_id == yard_id)
60	62		.order_by(CheckpointEvent.recorded_at.desc(), CheckpointEvent.id.desc())
61		-	.offset(offset)
62		-	.limit(limit)
	63	+	.offset(offset)
	64	+	.limit(limit)
63	65		)
64	66		rows = (await db.scalars(stmt)).all()
65		-	return CheckpointPage(items=rows, page=page, limit=limit)
	67	+	next_cursor = str(page + 1) if len(rows) == limit else None
	68	+	return CheckpointPage(items=rows, next_cursor=next_cursor, limit=limit)