Fix two gateway test failures: outbox lane scoping + SSE teardown race

- outbox: delete_message/message_info now match the exact lane first
  (a flat-lane delete/lookup with thread_id=None sees only frames with
  no thread_id) and fall back to the message_id across all lanes only
  when the exact lane matches nothing. Previously lane=None meant
  'any lane' in the first pass, so a flat-lane delete also removed
  same-id frames from threads (test expected 3 removed, got 4).

- http_server: the SSE live loop skipped queued frames when stop() set
  sub.closed before the handler thread reached the loop (descheduled
  under load between the initial hello/status writes and the loop).
  The loop now drains frames queued before the close, so the
  status{restarting} teardown broadcast always reaches the client
  before EOF (test_disconnect_broadcasts_status_restarting was flaky
  ~70% under CPU load).

gateway-plugin/http_server.py

@@ -523,7 +523,20 @@ class HttpServer:
             for snap in self._adapter.todo_snapshot_frames():
                 self._write_sse(handler, "frame", None, snap.to_json())
             # 3. Live frames (cursor=None frames have no id).
-            while not sub.closed.is_set():
+            while True:
+                if sub.closed.is_set():
+                    # stop() can land between the initial writes above and
+                    # this loop (the handler thread is descheduled under
+                    # load): drain the frames queued before the close — e.g.
+                    # the status{restarting} teardown broadcast — so the
+                    # client sees them before EOF instead of losing them to
+                    # the closed check.
+                    try:
+                        item = sub.q.get_nowait()
+                    except queue.Empty:
+                        reason = "stopped"
+                        break
+                else:
                     try:
                         item = sub.q.get(timeout=SSE_HEARTBEAT_S)
                     except queue.Empty:

gateway-plugin/outbox.py

@@ -303,10 +303,11 @@ class Outbox:
         frame of a streamed reply -- or ``None`` when the message is not in the
         outbox (e.g. already pruned by retention).
 
-        The lane is matched exactly first; when that finds nothing the lookup
+        The lane is matched exactly first (a flat-lane lookup, ``thread_id
-        falls back to the ``message_id`` alone (it is a unique uuid4), so a
+        = None``, sees only frames with no ``thread_id``); when that finds
-        stale/missing ``thread_id`` on the request still resolves the row.
+        nothing the lookup falls back to the ``message_id`` alone across all
-        (``lane=None`` in the scan means "any lane".)
+        lanes (it is a unique uuid4), so a stale/missing ``thread_id`` on the
+        request still resolves the row.
         """
         if not message_id:
             return None
@@ -315,7 +316,7 @@ class Outbox:
                 "SELECT frame FROM outbox WHERE chat_id = ?", (chat_id,)
             ).fetchall()
 
-        def scan(lane: str | None) -> dict[str, Any] | None:
+        def scan(lane: str | None, exact: bool) -> dict[str, Any] | None:
             msg_frame: dict[str, Any] | None = None
             stop_frame: dict[str, Any] | None = None
             for r in rows:
@@ -325,7 +326,7 @@ class Outbox:
                     continue
                 if not isinstance(frame, dict):
                     continue
-                if lane is not None and _frame_thread_id(frame) != lane:
+                if exact and _frame_thread_id(frame) != lane:
                     continue
                 payload = frame.get("payload")
                 if not isinstance(payload, dict) or payload.get("message_id") != message_id:
@@ -345,7 +346,7 @@ class Outbox:
                     }
             return msg_frame or stop_frame
 
-        return scan(thread_id) or scan(None)
+        return scan(thread_id, exact=True) or scan(None, exact=False)
 
     def delete_message(
         self,
@@ -376,7 +377,7 @@ class Outbox:
                 "SELECT cursor, frame FROM outbox WHERE chat_id = ?", (chat_id,)
             ).fetchall()
 
-            def cursors_for(lane: str | None) -> list[int]:
+            def cursors_for(lane: str | None, exact: bool) -> list[int]:
                 out: list[int] = []
                 for r in rows:
                     try:
@@ -385,14 +386,17 @@ class Outbox:
                         continue
                     if not isinstance(frame, dict):
                         continue
-                    if lane is not None and _frame_thread_id(frame) != lane:
+                    if exact and _frame_thread_id(frame) != lane:
                         continue
                     payload = frame.get("payload")
                     if isinstance(payload, dict) and payload.get("message_id") == message_id:
                         out.append(int(r["cursor"]))
                 return out
 
-            cursors = cursors_for(thread_id) or cursors_for(None)
+            # Exact lane first (a flat-lane delete must not reach into
+            # threads); fall back to the message_id across all lanes only
+            # when the exact lane matches nothing (stale/missing thread_id).
+            cursors = cursors_for(thread_id, exact=True) or cursors_for(thread_id, exact=False)
             if not cursors:
                 return 0
             # One bound-parameter delete per cursor (a message spans only a few