feat(sp25): /api/acks/stream + /api/ta1-acks/stream NDJSON endpoints

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.

backend/src/cyclone/api_routers/acks.py

@@ -1,4 +1,4 @@
-"""``/api/acks`` — list & detail endpoints for the 999 ACK inbox.
+"""``/api/acks`` — list, detail, and live-tail stream for 999 ACKs.
 
 These are the persisted acknowledgment rows produced by
 ``POST /api/parse-999``. The frontend ``useAcks`` hook re-shapes the
@@ -7,18 +7,29 @@ list payload to its ``Ack`` interface in ``src/types/index.ts``.
 The detail endpoint returns the full ``raw_json`` payload plus the
 regenerated ``raw_999_text`` so the UI can show "view source" without a
 second round-trip.
+
+SP25: ``/api/acks/stream`` joins the live-tail triplet — the Acks
+page mounts ``useTailStream("acks")`` and ``useMergedTail("acks", …)``
+to see new 999 acks the moment they land (whether from the SFTP
+poller or a manual upload).
 """
 from __future__ import annotations
 
 import logging
-from typing import Any
+from typing import Any, AsyncIterator
 
 from fastapi import APIRouter, HTTPException, Query, Request
 from fastapi.responses import StreamingResponse
 
-from cyclone.api_helpers import ndjson_stream_list, wants_ndjson
+from cyclone.api_helpers import (
+    ndjson_line,
+    ndjson_stream_list,
+    tail_events,
+    wants_ndjson,
+)
 from cyclone.parsers.models_999 import ParseResult999
 from cyclone.parsers.serialize_999 import serialize_999
+from cyclone.pubsub import EventBus
 from cyclone.store import store, to_ui_ack
 
 router = APIRouter()
@@ -71,6 +82,35 @@ def list_acks_endpoint(
     }
 
 
+@router.get("/api/acks/stream")
+async def acks_stream(
+    request: Request,
+    limit: int = Query(100, ge=1, le=1000),
+) -> StreamingResponse:
+    """Stream 999 ACKs as NDJSON: snapshot first, then live events.
+
+    SP25: this endpoint joins the live-tail triplet — subscribes to
+    ``ack_received`` and emits one ``item`` per snapshot row plus a
+    single ``snapshot_end`` line, then forwards live events from
+    the bus. Matches the wire format used by ``/api/claims/stream``,
+    ``/api/remittances/stream``, and ``/api/activity/stream``.
+
+    NOTE: registered BEFORE ``/api/acks/{ack_id}`` so the literal
+    ``stream`` path segment doesn't get matched as an ack id.
+    """
+    bus: EventBus = request.app.state.event_bus
+
+    async def gen() -> AsyncIterator[bytes]:
+        rows = store.list_acks()[:limit]
+        for row in rows:
+            yield ndjson_line({"type": "item", "data": to_ui_ack(row)})
+        yield ndjson_line({"type": "snapshot_end", "data": {"count": len(rows)}})
+        async for chunk in tail_events(request, bus, ["ack_received"]):
+            yield chunk
+
+    return StreamingResponse(gen(), media_type="application/x-ndjson")
+
+
 @router.get("/api/acks/{ack_id}")
 def get_ack_endpoint(ack_id: int) -> dict:
     """Return one persisted ACK row with its parsed detail.

backend/src/cyclone/api_routers/ta1_acks.py

@@ -1,4 +1,4 @@
-"""``/api/ta1-acks`` — list & detail endpoints for persisted TA1 envelopes.
+"""``/api/ta1-acks`` — list, detail, and live-tail stream for TA1 envelopes.
 
 TA1 is the interchange-control ACK (ISA/IEA acknowledgement). It's a
 single segment, no functional group, no transaction set. Cyclone
@@ -7,14 +7,21 @@ row can sit alongside the 999 / 277CA ack rows without special-casing.
 
 The detail endpoint also reconstructs the TA1 segment string
 (``TA1*...~``) so the operator can copy it into a downstream tool.
+
+SP25: ``/api/ta1-acks/stream`` joins the live-tail triplet — the
+Acks page mounts ``useTailStream("ta1_acks")`` so the TA1 envelope
+ack section sees new rows the moment they land.
 """
 from __future__ import annotations
 
-from typing import Any
+from typing import Any, AsyncIterator
 
-from fastapi import APIRouter, HTTPException, Query
+from fastapi import APIRouter, HTTPException, Query, Request
+from fastapi.responses import StreamingResponse
 
+from cyclone.api_helpers import ndjson_line, tail_events
 from cyclone import db
+from cyclone.pubsub import EventBus
 from cyclone.store import store, to_ui_ta1_ack
 
 router = APIRouter()
@@ -52,6 +59,30 @@ def list_ta1_acks_endpoint(
     }
 
 
+@router.get("/api/ta1-acks/stream")
+async def ta1_acks_stream(
+    request: Request,
+    limit: int = Query(100, ge=1, le=1000),
+) -> StreamingResponse:
+    """Stream TA1 envelope acks as NDJSON.
+
+    Subscribes to ``ta1_ack_received`` and emits the same wire format
+    as the other live-tail endpoints. Registered BEFORE
+    ``/api/ta1-acks/{ack_id}`` so ``stream`` isn't matched as an id.
+    """
+    bus: EventBus = request.app.state.event_bus
+
+    async def gen() -> AsyncIterator[bytes]:
+        rows = store.list_ta1_acks()[:limit]
+        for row in rows:
+            yield ndjson_line({"type": "item", "data": to_ui_ta1_ack(row)})
+        yield ndjson_line({"type": "snapshot_end", "data": {"count": len(rows)}})
+        async for chunk in tail_events(request, bus, ["ta1_ack_received"]):
+            yield chunk
+
+    return StreamingResponse(gen(), media_type="application/x-ndjson")
+
+
 @router.get("/api/ta1-acks/{ack_id}")
 def get_ta1_ack_endpoint(ack_id: int) -> dict:
     """Return one persisted TA1 ACK row with its parsed detail."""