docs: update documentation with chat enhancements

Document three recent chat improvements:

1. Real-time Active Users Fix (dd31761)
   - Fixed bug where users list didn't update when new users joined
   - Rewrote getAllDisplayUsers() to prioritize Socket.IO data

2. Message Length Limits (4a91a10)
   - Added 2000 character limit with visual counter
   - Backend + frontend validation

3. Spam Protection System (ace3311)
   - Rate limiting: 10 messages per minute
   - Duplicate detection within 60 second window
   - Profanity filter with Polish + English words

Updated:
- README.md: Added chat features to Events & Chat section
- SESSION_CONTEXT.md: New "Chat Enhancements" section
- COMPLETED.md: Comprehensive entry with problem/solution/impact
- Last updated dates: 2025-12-03

README.md

@@ -29,9 +29,13 @@ Web application (PWA) enabling dance event participants to:
 ### Events & Chat
 - Event list from worldsdc.com
 - Real-time event chat (Socket.IO) with active users sidebar
+- Real-time active users list (instant updates when users join/leave)
 - Infinite scroll message history
 - Clickable usernames (/@{username}) with country flags
 - Competitor numbers (bib numbers) display
+- Message validation: 2000 character limit with visual counter
+- Spam protection: rate limiting (10 msg/min), duplicate detection, profanity filter
+- Polish + English profanity filtering
 
 ### Auto-matching & Fairness System
 - Smart recording assignment for competition heats
@@ -204,7 +208,7 @@ spotlightcam/
 │   │   ├── controllers/       # Auth, users, events, WSDC
 │   │   ├── routes/            # API routes (events, matches, admin)
 │   │   ├── services/          # Matching algorithm, activity logging
-│   │   ├── middleware/        # Auth, admin access control
+│   │   ├── middleware/        # Auth, admin access, message validation (spam protection)
 │   │   ├── socket/            # Socket.IO handlers (chat, WebRTC signaling, admin logs)
 │   │   └── __tests__/         # Jest tests (342 tests)
 │   ├── prisma/
@@ -376,4 +380,4 @@ TBD
 ---
 
 **Status:** MVP Complete ✅ | 342/342 tests passing (100%) | Production Ready
-**Last Updated:** 2025-12-02
+**Last Updated:** 2025-12-03