Implement WOPI integration for Collabora Online
- Add WOPI models (CheckFileInfoResponse, PutFileResponse, LockInfo)
- Implement WOPI handlers: CheckFileInfo, GetFile, PutFile, Lock operations
- Add file locking mechanism to prevent concurrent editing conflicts
- Add WOPI session endpoint for generating access tokens
- Add UpdateFileSize method to database
- Add WOPI routes (/wopi/files/* endpoints)
- Update Flutter document viewer to load Collabora via WOPI WOPISrc URL
- Implement WebView integration for Collabora Online viewer
- Add comprehensive logging for WOPI operations [WOPI-TOKEN], [WOPI-REQUEST], [WOPI-STORAGE], [WOPI-LOCK]
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