v3.1.0: Invite-gated auth, Glicko-2 ratings, matchmaking queue

- Enforce invite codes on registration (INVITE_ONLY=true by default)
- Bootstrap admin account for first-time setup
- Require authentication for WebSocket connections and room creation
- Add Glicko-2 rating system with multiplayer pairwise comparisons
- Add Redis-backed matchmaking queue with expanding rating window
- Auto-start matched games with standard rules after countdown
- Add "Find Game" button and matchmaking UI to client
- Add rating column to leaderboard
- Scale down docker-compose.prod.yml for 512MB droplet

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

client/leaderboard.js

@@ -26,6 +26,7 @@ class LeaderboardComponent {
             avg_score: 'Avg Score',
             knockouts: 'Knockouts',
             streak: 'Best Streak',
+            rating: 'Rating',
         };
 
         this.metricFormats = {
@@ -34,6 +35,7 @@ class LeaderboardComponent {
             avg_score: (v) => v.toFixed(1),
             knockouts: (v) => v.toLocaleString(),
             streak: (v) => v.toLocaleString(),
+            rating: (v) => Math.round(v).toLocaleString(),
         };
 
         this.init();