docs: add AGENTS.md for realtime module

Author: robinp7720

src/lib/realtime/AGENTS.md

@@ -0,0 +1,10 @@
+# Realtime Socket Interface (src/lib/realtime/)
+
+What lives here
+- `RealtimeController.ts`: Manages the Socket.IO server and client connections.
+- `RealtimeClient.ts`: Handles individual client socket sessions, authentication, and event processing.
+
+Notes
+- This module implements the Realtime API documented in `docs/REALTIME_API.md`.
+- **CRITICAL:** If you modify the events, payloads, or authentication flow in this directory, you **MUST** update `docs/REALTIME_API.md` to reflect these changes.
+- The `playing` event updates are buffered in `RealtimeClient` and saved to the database every 10 seconds via `saveAllTracks`.