feat: implement OnSharedChannelsSyncMsg hook for bidirectional sync

- Add PublishMessage method to BridgeManager interface and implementation
- Implement OnSharedChannelsSyncMsg hook to process Mattermost shared channel sync messages
- Add processSyncPost helper to convert Mattermost posts to bridge messages
- Route sync messages from Mattermost shared channels to XMPP bridge via message bus
- Handle user resolution with fallback to API calls for missing users
- Add comprehensive logging and error handling for sync operations
- Support routing of text messages from shared channels to XMPP rooms

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

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

server/model/bridge.go

@@ -120,6 +120,9 @@ type BridgeManager interface {
 
 	// DeleteChannepMapping is called when a channel mapping is deleted.
 	DeleteChannepMapping(req DeleteChannelMappingRequest) error
+
+	// PublishMessage publishes a message to the message bus for routing to target bridges
+	PublishMessage(msg *DirectionalMessage) error
 }
 
 type Bridge interface {