Protected Tags dont protect if delete happens via WebUI

[6543]

Gitea Version

v1.16.x (codeberg.org); up to main (460c03c)

Reproduce

1. create a tag
2. protect this tag
3. delete via webui

expect

A. no delete option for protected tags
B. an error if backend gets delete request

[6543]

also if you like to create a tag witch is protected via UI you get an 500 page

a flash error would be a better UX