feat: implement bidirectional message bridge system with XMPP-Mattermost integration

This commit implements a comprehensive bridge-agnostic message routing system that enables
real-time bidirectional message synchronization between XMPP and Mattermost platforms.

Key features:
- Bridge-agnostic message types and structures for extensibility
- Central message bus system with publisher-subscriber pattern
- Complete Bridge interface implementation for both XMPP and Mattermost
- Message aggregation from multiple sources for scalability
- Loop prevention mechanisms to avoid infinite message cycles
- Buffered channels for high-performance message processing

Architecture highlights:
- Producer-consumer pattern for message routing between bridges
- Thread-safe goroutine lifecycle management with context cancellation
- Message handlers separated into dedicated files for maintainability
- Support for future bridge implementations (Slack, Discord, etc.)
- Markdown content standardization across all bridges

Files added:
- server/model/message.go: Core bridge-agnostic message structures
- server/bridge/messagebus.go: Central message routing system
- server/bridge/mattermost/message_handler.go: Mattermost-specific message processing
- server/bridge/xmpp/message_handler.go: XMPP-specific message processing

Files modified:
- server/bridge/manager.go: Integration with message bus and routing
- server/bridge/mattermost/bridge.go: Complete Bridge interface implementation
- server/bridge/xmpp/bridge.go: Message aggregation and interface completion
- server/model/bridge.go: Extended Bridge interface for bidirectional messaging
- server/xmpp/client.go: Enhanced message listening with mellium.im/xmpp

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

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

server/model/bridge.go

@@ -152,6 +152,12 @@ type Bridge interface {
 
 	// GetUserManager returns the user manager for this bridge.
 	GetUserManager() BridgeUserManager
+
+	// Message handling for bidirectional communication
+	GetMessageChannel() <-chan *DirectionalMessage
+	SendMessage(msg *BridgeMessage) error
+	GetMessageHandler() MessageHandler
+	GetUserResolver() UserResolver
 }
 
 // BridgeUser represents a user connected to any bridge service