fix(wopi): use fresh storage mtime for conflict detection and responses

[elzody]

Resolves:

• WOPI timestamp comparison uses stale modification time causing false DOC_CHANGED conflicts #5832
• Send reliable timestamp for when "Document changed in storage" conflicts happen #5794

Target version: main

Summary

Apparently, in some cases when multiple people are editing a document, especially in longer editing sessions, the mtime of the file in the oc_filecache table can become out of sync (behind) what the mtime is on the file storage, leading to a conflict. Re-scanning files on an instance via occ files:scan resolves the conflict as the file cache's mtime is updated to match the storage's.

I have not been able to reproduce the issue, but hopefully reading the mtime from the storage itself helps instead of relying on a potentially outdated one. Additionally, it includes the LastModifiedTime in the conflict dialog.

Assisted-by: ClaudeCode:claude-sonnet-4-6

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required