Remove a tag from a file

Remove a specific tag from a file.

Behavior:

• Removes the specified tag from the document
• Works for both manually assigned and auto-assigned tags
• Idempotent: returns 204 even if tag was not on the document

Validation:

• Returns 403 if user doesn't have permission to edit the document
• Returns 404 if document doesn't exist
• Returns 404 if tag doesn't exist or doesn't belong to user's company