feat(sync): add canonical room media state

shared/README.md

@@ -31,6 +31,7 @@ Browser extensions cannot import files outside their own root directory, so the
 - `CONTROL_MODES.HOST_ONLY`: only the host and promoted controllers may move playback.
 - `CAPABILITIES.HOST_CONTROL`: relay supports host-only room authority.
 - `CAPABILITIES.CO_HOST`: relay supports promoted controller peers.
+- `CAPABILITIES.MEDIA_STATE_V1`: relay exposes canonical room playback recovery snapshots.
 
 Clients should enable capability-gated UI only when the relay advertises the matching flag in `room_data.capabilities`.
 
@@ -64,6 +65,7 @@ For the complete event list, read the `EVENTS` object in [`constants.js`](consta
 - `HEARTBEAT_INTERVAL`: content heartbeat interval in milliseconds.
 - `FORCE_SYNC_TIMEOUT`: max wait for force-sync ACKs.
 - `EPISODE_LOBBY_TIMEOUT`: max wait for episode-lobby readiness.
+- `MAX_MEDIA_TIME`: shared relay/extension upper bound for synchronized media positions.
 
 ## Do Not Break
 

shared/constants.js

@@ -82,9 +82,13 @@ export const CAPABILITIES = {
     HOST_CONTROL: 'host-control',
     CO_HOST: 'co-host',  // owner promotes guests to additional controllers
     CHAT: 'chat',        // legacy server capability used by the first chat beta
-    CHAT_V1: 'chat-v1'   // versioned client/server chat wire contract
+    CHAT_V1: 'chat-v1',  // versioned client/server chat wire contract
+    MEDIA_STATE_V1: 'media-state-v1' // server-authoritative room playback recovery snapshot
 };
 
+// Relay and extension media-time validation must use the same upper bound.
+export const MAX_MEDIA_TIME = 86400;
+
 export const HEARTBEAT_INTERVAL = 15000; // 15s
 export const FORCE_SYNC_TIMEOUT = 8500; // 8.5s timeout for force sync ACKs (must be > content.js poll timeout of 8s)
 export const EPISODE_LOBBY_TIMEOUT = 60000; // 60s timeout for episode lobby