fix(richdocuments): gate conversion with SecureViewService check

[chrip]

Server-side conversion bypassed the Secure View / watermark restriction that the viewer enforces, allowing a user with view-only secure access to download a clean copy via the conversion API. Reuse SecureViewService (same logic the viewer uses) to deny conversion for files that should be secured. Handle the documented NotFoundException so a cache miss surfaces as a clear, translated error instead of a 500.

• Resolves: #
• Target version: main

Summary

TODO

• [ x] If the file is secured, the "Save as..." is not possible because this could bypass the security. We could hide the menu entry in this case. Would need changes in apps/files/src/actions/convertAction.ts

Checklist

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

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