Add multi-user server sync (PHP server + client)

Phase 2 implementation: Multiple streamers can now merge their captions
into a single stream using a PHP server.

PHP Server (server/php/):
- server.php: API endpoint for sending/streaming transcriptions
- display.php: Web page for viewing merged captions in OBS
- config.php: Server configuration
- .htaccess: Security settings
- README.md: Comprehensive deployment guide

Features:
- Room-based isolation (multiple groups on same server)
- Passphrase authentication per room
- Real-time streaming via Server-Sent Events (SSE)
- Different colors for each user
- File-based storage (no database required)
- Auto-cleanup of old rooms
- Works on standard PHP hosting

Client-Side:
- client/server_sync.py: HTTP client for sending to PHP server
- Settings dialog updated with server sync options
- Config updated with server_sync section

Server Configuration:
- URL: Server endpoint (e.g., http://example.com/transcription/server.php)
- Room: Unique room name for your group
- Passphrase: Shared secret for authentication

OBS Integration:
Display URL format:
http://example.com/transcription/display.php?room=ROOM&passphrase=PASS&fade=10&timestamps=true

NOTE: Main window integration pending (client sends transcriptions)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

config/default_config.yaml

@@ -25,8 +25,9 @@ processing:
 
 server_sync:
   enabled: false
-  url: "ws://localhost:8000"
-  api_key: ""
+  url: "http://localhost/transcription/server.php"
+  room: "default"
+  passphrase: ""
 
 display:
   show_timestamps: true