Add Steam integration showing online status and recent games

Fetches player summary and recently played games from Steam API with
5-minute server-side caching. Displays in the home sidebar with online
indicator and game artwork.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/graph/schema/schema.graphql

@@ -12,6 +12,7 @@ type Query {
   spotifyListening: SpotifyPlaying
   spotifyRecent: [SpotifyRecentItem!]
   giteaFeed: GiteaFeedItem
+  steamStatus: SteamStatus
   me: User
 }