Adds two streaming endpoints that match the live-tail wire format
established by /api/claims/stream, /api/remittances/stream, and
/api/activity/stream:
* /api/acks/stream — subscribes to ack_received
* /api/ta1-acks/stream — subscribes to ta1_ack_received
Both yield a snapshot of existing rows (newest first, capped by the
`limit` query param), then a `snapshot_end` line, then forward
live events from the bus. They are registered before the
/{ack_id} path-param endpoints so the literal `stream` segment
isn't matched as an id.
Tests use the same direct-coroutine pattern as test_api_stream_live.py
because httpx.ASGITransport buffers the response body and never
delivers a disconnect message — iterating body_iterator directly
with body_iterator.aclose() simulates a client disconnect.
Nora