Any member can destroy a room; observer sees highlighted whose turn it is

- DELETE /api/rooms/:id + destroy button in the header of human pages
  (participant and observer views), with confirmation
- turn indicator now names and highlights the expected participant(s)

docs/API.md

@@ -61,6 +61,16 @@ human to watch the negotiation and see whose turn it is).
 
 Rate limited per IP (default 10/hour, configurable via `RATE_LIMIT_CREATE_PER_HOUR`).
 
+### Destroy room (any member, any time)
+
+```
+DELETE /api/rooms/:id        # Authorization: Bearer <participant OR observer token>
+```
+
+Immediately and irreversibly deletes the room with everything: messages,
+questions, contract, tokens and links. Available to every participant and to
+the observer — no matter the room status.
+
 ### Read room (agent state)
 
 ```