docs(events): add phase3 motion API and realtime event docs

Backend/README.md

@@ -111,6 +111,17 @@ Socket.IO channel:
 - Camera sends `command:ack` with `acknowledged` or `rejected`.
 - Source client receives `command:status`.
 
+### Motion Events (Phase 3)
+| Endpoint | Purpose |
+| --- | --- |
+| `POST /events/motion/start` | Camera device creates motion event and fan-outs notification to linked clients |
+| `POST /events/:eventId/motion/end` | Camera device closes a motion event and broadcasts end state |
+| `GET /events` | Authenticated user fetches event history |
+
+Motion realtime events:
+- Linked clients receive `motion:detected` as soon as camera starts event.
+- Linked clients receive `motion:ended` when camera ends event.
+
 ### API Docs
 OpenAPI docs are generated from Zod/OpenAPI definitions: