Topic admin APIs

Base path: /api/server/v1. The :topic segment is the logical topic (orders), not the events: prefix.

Client method	HTTP	Notes
`setTopicState(topic, state)`	`PUT /topics/:topic/state`	Body is the JSON object to store.
`getTopicState(topic)`	`GET /topics/:topic/state`	Returns the `state` map.
`getTopicStateWithMeta(topic)`	`GET /topics/:topic/state`	Adds `sourceNode` / `servedByNode` from `meta`.
`clearTopicState(topic)`	`DELETE /topics/:topic`	Returns boolean `closed`.
`sendTopicMessage(topic, { event, payload? })`	`POST /topics/:topic/messages`	Server responds with `202 Accepted`.
`getTopicPresence(topic)`	`GET /topics/:topic/presence`	Maps JSON `client_count` to `clientCount`.

curl -X PUT "http://localhost:4000/api/server/v1/topics/orders/state" \
  -H 'content-type: application/json' \
  -d '{"count":1}'

await admin.setTopicState("orders", { count: 1 });
await admin.sendTopicMessage("orders", { event: "created", payload: { orderId: "o1" } });