Stream each task's transcript over its own SSE; keep session note order

api/openapi.yaml

@@ -502,15 +502,18 @@ paths:
       description: >
         Server-sent events. Each event's id is its seq; data is the Event as JSON.
         Events after Last-Event-ID (or the after query parameter) are replayed before live events.
+        With task_id, only that task's events are sent.
       parameters:
         - { name: Last-Event-ID, in: header, required: false, schema: { type: integer, format: int64 } }
         - { name: after, in: query, required: false, schema: { type: integer, format: int64 } }
+        - { name: task_id, in: query, required: false, schema: { type: string } }
       responses:
         "200":
           description: Event stream.
           content:
             text/event-stream:
               schema: { type: string }
+        "404": { $ref: "#/components/responses/Error" }
 
   /api/admin/status:
     get: