feat: implement WebRTC P2P file transfer with DataChannel

Add complete WebRTC peer-to-peer file transfer functionality:

Backend changes:
- Add WebRTC signaling events to Socket.IO (offer, answer, ICE candidates)
- Implement authorization checks for match participants
- Add signaling relay between matched users

Frontend changes:
- Create useWebRTC hook for RTCPeerConnection management
- Implement RTCDataChannel with 16KB chunking for large files
- Add real-time progress monitoring for sender and receiver
- Implement automatic file download on receiver side
- Add connection state tracking and error handling
- Integrate WebRTC with MatchChatPage (replace mockup)

Configuration:
- Add Vite allowed hosts configuration via VITE_ALLOWED_HOSTS env var
- Support comma-separated host list or 'all' for development
- Add .env.example with configuration examples
- Update docker-compose.yml with default allowed hosts

Documentation:
- Add comprehensive WebRTC testing guide with troubleshooting
- Add quick test checklist for manual testing
- Document WebRTC flow, requirements, and success criteria

Features:
- End-to-end encrypted P2P transfer (DTLS)
- 16KB chunk size optimized for DataChannel
- Buffer management to prevent overflow
- Automatic connection establishment with 30s timeout
- Support for files of any size
- Real-time progress tracking
- Clean connection lifecycle management

QUICK_TEST.md

@@ -0,0 +1,71 @@
+# Quick WebRTC Test Checklist
+
+## Setup (2 browser windows)
+
+**Window 1:** Login as `john@example.com` / `Dance123!`  
+**Window 2:** Login as `sarah@example.com` / `Swing456!`
+
+## Test Steps
+
+### 1. Create Match
+- [ ] User A: Go to event → Request match with User B
+- [ ] User B: Accept match
+- [ ] Both: Navigate to match chat
+
+### 2. Establish WebRTC Connection
+- [ ] User A: Click "Send video (WebRTC)"
+- [ ] User A: Select a small video file (~5-10MB)
+- [ ] User A: Click "Send video (P2P)"
+- [ ] Both: Status shows "Connecting..." → "Connected (P2P)" ✅
+
+### 3. File Transfer
+- [ ] User A: See progress bar 0% → 100%
+- [ ] User B: See "📥 Receiving: [filename]"
+- [ ] User B: File downloads automatically when complete
+- [ ] Both: Chat message appears: "📹 Video sent: [filename]"
+
+## Console Logs to Check (F12)
+
+**User A:**
+```
+📤 Sent WebRTC offer
+📤 Sent ICE candidate
+✅ DataChannel opened
+📤 Sent file metadata
+📤 Sent chunk 1/X
+✅ File transfer complete
+```
+
+**User B:**
+```
+📥 Received WebRTC offer
+✅ DataChannel received
+📥 Receiving file
+📥 Received chunk 1/X
+✅ File received and downloaded
+```
+
+## Success Criteria
+
+✅ Connection state: "Connected (P2P)" with green dot  
+✅ Transfer completes: 100% on both sides  
+✅ File downloads on receiver side  
+✅ File size matches original  
+✅ No errors in console
+
+## If Something Fails
+
+1. Check both users are in same match chat
+2. Check Socket.IO is connected (message input not disabled)
+3. Check browser console for errors
+4. Try refreshing both windows
+5. See WEBRTC_TESTING_GUIDE.md for detailed troubleshooting
+
+## Start Testing
+
+```bash
+docker compose up --build
+# Then open http://localhost:8080 in two browser windows
+```
+
+🚀 **Ready to test!**