Hosted rooms' instance IDs name their owner: <owner>-<2100000000+n>

The client takes a room's owner from the instance ID's first part; with 2100000000 there, the owner
couldn't decorate their hosted room (putting furniture on went to their home room instead).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/api/test/hosted.test.ts

@@ -133,7 +133,7 @@ test("rooms are made, listed, changed and closed by their owners", async () => {
   expect(made.status).toBe(201);
   const n = made.json.room.n;
   expect(made.json.room).toMatchObject({
-    instanceId: `2100000000-${n}`,
+    instanceId: `302-${2_100_000_000 + n}`,
     chatId: 2_100_000_000 + n,
     name: "The Lounge",
     description: "A quiet place.",